CINESCOPUS IMPELLER APPARATUS
This invention relates generally to amplifiers, and in particular to an apparatus for providing amplification of a video signal for driving the cathode electrode of a kinescope. In a television apparatus employing direct vision or projection kinescopes as visual display devices, it is desirable that the amplifier driving the cathode of the kinescope provide a relatively high voltage impulse signal having a broadband amplitude and a high oscillation index. Typically, the pulse voltages may be of the order of about 200 volts, the band amplitudes may be 5 MHz or higher, and the oscillation rates may be substantially greater than 100 volts / microsecond. To facilitate high voltage operation, it is common to employ a cascade configuration of a common emitter input stage that drives a common battery output stage. This configuration requires only one high voltage transistor (the output stage), and since it is connected in a common base configuration, the Miller effect is suppressed, and therefore, a very high bandwidth operation is possible. wide In practice, the actual bandwidth and oscillation index that can be achieved in a cascade amplifier depend, to a degree, on the effective load capacitance presented to the output stage and the available output current. In general, the operating current of the amplifier can be increased, or the effective load capacitance can be decreased to maximize the amplitude of the band and the oscillation index of the amplifier. However, since the increase in current necessarily implies increasing the energy dissipation of the amplifier, it is preferable to take measures to reduce the effective load capacitance in order to have a better operation, rather than resorting to increases in operating energy. In the applications of kinescope boosters, the "effective" charge capacitance presented to the amplifier is mainly that of the kinescope cathode and the parasitic capacitances associated with the socket, the spark gap, the wiring, and the like. An effective approach to reduce the load of the effective capacitance is to couple the amplifier with the cathode by means of a complementary emitter follower amplifier in push-pull. This amplifier effectively "isolates" the load capacitance approximately in proportion to the reciprocal of the current gain of the transistor ("beta"). The additional current provided by the follower amplifier provides a faster charge and discharge of the load capacitance, and therefore, improves the oscillation index and the bandwidth. To avoid substantially increasing the power dissipation it is a common practice to operate the follower amplifier in a "class B" mode where the push-pull transistors are polarized to avoid simultaneous driving. It has been recognized by John H. Furrey, in U.S. Patent Number 4,860,107 entitled VIDEO DISPLAY DRIVER APPARATUS, which issued on August 22, 1989, that a better capacitance reduction can be obtained by utilizing a "series form" of push-pull complementary emitter follower, rather than the more ordinary "parallel shape" of push-pull complementary emitter follower, and Furrey developed this amplifier with the desired class B driving operation. In more detail, by definition, as used herein a "parallel form" of complementary emitter (or source) follower, is one where the inputs (base or gate electrodes) of a pair of complementary transistors (bipolar) or field effect) are connected in parallel to receive an input signal from an amplifier, and the outputs (emitters or sources) are connected in parallel to drive a load. The term "serial form" of the issuer follower or complementary source, is one in which the complementary emitter (or source) followers are connected in series to form a cascade connection between the output of an amplifier and a load, and that includes diodes to bypass the serial transistor that is not driving the load . In the Furrey complementary "push-pull" emitter follower series form, diodes are provided for each transistor to derive to the transistor that is depolarized. Specifically, a diode is connected through the junction of the base emitter of each transistor, and each diode is biased for forward current conduction in a direction opposite to that of the associated base emitter junction. This significantly reduces the effective load capacitance of the visual display (load capacitances and kinetic parasites), thereby improving the transient response of the positive and negative video signal. The use of complementary push-pull emitter amplifier amplifiers (or more generally, "voltage follower"), as described, is effective in decreasing the capacitance presented to the impeller amplifier that can be attributed to the capacitance associated with the cathode. of the kinescope. However, it is recognized herein that a further improvement in the impeller amplifier can still be obtained when any type (i.e., "in series" or "parallel" type) of complementary emitter follower amplifier is used in contraphase, to couple the output of the impeller with the charge of the cathode electrode. The present invention is directed to the satisfaction of this need. Also, the principles of the invention can be extended to single-ended impeller applications, as will be explained. The kinescope drive apparatus incorporating the invention comprises a video amplifier having an output coupled to an electrode of the kinescope cathode by means of a voltage follower. The voltage follower comprises a transistor having a conduction path and a control electrode for controlling the conduction of the path, the control electrode coupling to receive a video signal from the video amplifier, coupling one end of the path of conduction with a point of reference potential by means of a current source, and coupling to the kinescope cathode, and coupling the second end of the conduction path with a source of supply voltage. A feedback loop is provided to apply a positive feedback voltage to the second end of the conduction path of the voltage follower transistor to maintain a substantially constant voltage across the conduction path, the voltage being substantially constant independent of the variations in voltage. the video signal applied to the control electrode. The foregoing and other features of the invention are illustrated in the accompanying drawing, in which: Figure 1 is a schematic diagram, partly in the form of blocks, of a visual television display apparatus embodying the invention. Figure 2 illustrates a modification of the apparatus of Figure 1, wherein a high voltage impeller portion is simplified for the forward feed operation, and wherein the positive phase shift feedback circuitry is simplified to reduce the count of the active device. . Fig. 3 illustrates a modification of the apparatus of Fig. 1, wherein the supplementary counter-phaser emitter damping amplifiers of the parallel type are replaced by complementary push-pull emitter damping amplifiers of a serial type. Figure 4 illustrates a modification of the apparatus of Figure 2, wherein the complementary push-pull emitter amplifier amplifiers of the series type are replaced by complementary push-pull emitter amplifiers of the series type.
Figure 5 is a circuit diagram, partially in the form of blocks, illustrating a modification of the example of Figure 2 for single-ended kinescope impeller applications. Figure 6 illustrates a further modification of the example of Figure 1. Before considering the details of the television system of Figure 1, it is useful to first consider in greater detail the problem of using conventional push-pull emitter damping amplifiers to isolate the kinescope cathode capacitance of the output of the kinescope impeller amplifier. As explained above, the use of a follower amplifier is effective to reduce the capacitance that can be attributed to the cathode (and the associated parasites) that occurs at the output of the high voltage video boost amplifier. However, in the present it is recognized that the follower amplifier itself can introduce a capacitance load effect in the impeller amplifier, and this will tend to limit the overall operation of the system. In more detail, it has been found that the main source of undesirable capacitance loading effects in the kinescope drive systems of the type using push-pull followers, can be attributed to the collector capacitances at the base of the follower output transistors . Typically, these capacitances are smaller than the capacitance of the cathode of the kinescope, and in this way, the isolation of the cathode by means of a follower amplifier provides an overall reduction of capacitance and an improvement in the oscillation index and bandwidth, comparing with direct coupling systems. However, in order to achieve maximum benefit from the use of the emitter follower insulation, it is now recognized that it is desirable to reduce the effective capacitance of the follower amplifier itself. In order to achieve an effective reduction in the capacitance of the tracker, according to the present invention, feedback is used in such a way that the current flow under dynamic signal conditions in the capacitances of the collector is reduced to the base of the transistors of the tracker. This is achieved, as will be explained in more detail below, by applying the feedback in such a way as to maintain a collector voltage to the emitter substantially constant for the transistors of the follower. In turn, this tends to keep the collector voltage to the base constant. As a result, under dynamic signal conditions, there will be little or no charge or discharge of collector capacitances to the base as the signal voltage varies. The effective reduction in the input capacitance of the follower that can be attributed to the capacitances of the collector from the transistor to the base, is a function of the percentage of feedback applied to the regulation of the collector voltage to the emitter. For example, if the feedback percentage is selected in such a way that the variations of the collector voltage to the emitter are reduced by 50 percent, then the reactive currents that charge and discharge the collector capacitances to the base of the follower amplifier are also they will reduce by .50 percent, and in this way, the "effective" capacitance load will be cut in half. Greater reductions in the capacitance of the tracker can be achieved as the percentage of feedback to the unit increases. In the examples of the invention that follow, the percentage of positive feedback applied approaches 100 percent. For purposes of circuit stability, arrangements are made to ensure that the feedback gain can not be equal to or exceed the unit. In the illustrated examples, this is achieved by connecting all of the "active" semiconductor devices in the feedback paths in the voltage follower or "emitter" configurations. The above overview of the principles of the invention will now be described in detail with respect to the example of Figure 1, which illustrates a visual television display system that includes a video signal source 10 for supplying an SI video signal to a kinescope cathode 16 for visual display. To simplify the drawing, the details of the kinescope and the source of the signal are not shown. It will be appreciated that for a color system, there would be three impeller amplifiers. As an overview, to amplify the video signal up to the high voltage levels required at the cathode 16, the system includes a cascade type of high voltage amplifier 20 (illustrated in phantom). To isolate the output of the high voltage amplifier 20 from the capacitance of the cathode of the kinescope 16, the output of the amplifier 20 (collector of the transistor Q3) is coupled to the cathode 16 by means of a follower amplifier of the complementary emitter in the push-pull 30 (illustrated in ghost). To protect the drive amplifier from the kinescope arcs, the output terminal of the follower 15 is coupled to the cathode 16 by means of a kinescope arc protection resistor R15 and an inductor Ll. To provide a kinescope automatic polarization (AKB) operation, a cathode current sensing circuit 40 ("Ik detection", illustrated in phantom) is provided, which detects the collector current of a PNP transistor (Q7) in the follower amplifier of the push-pull emitter 30, for generating an output signal of automatic biasing of the kinescope at an output terminal 18, proportional to the cathode current, Ik, of the kinescope cathode 16. This characteristic is optional and can be omitted , as described later. Finally, to reduce the effective capacitance presented to the high voltage amplifier, which can be attributed to the collector capacitances to the base of the complementary emitter follower 30, the system includes a feedback control circuit 50 (illustrated in phantom) that maintains a substantially constant collector emitter voltage for the NPN transistor Q4 of the follower 30, and another feedback control circuit 60 (illustrated in phantom) that maintains a substantially constant collector emitter voltage for the PNP transistor Q7 of the follower 30. As noted above, and as explained in more detail below, the operation of the follower transistors in constant values from the collector voltage to the emitter also tends to regulate the collector voltage to the base at a nearly constant value, and this In turn, it tends to reduce the magnitude of the load and discharge currents of collector capacitances the base of the follower transistors. The beneficial result is that, since the driving amplifier 20 does not have to supply charging and discharging currents for these "parasitic" capacitances, the overall oscillation index, the bandwidth, and the transient response characteristics are improved. Now, consideration will be given to the details of the circuit and other operational features of the video display system of Figure 1. The signal source 10 can be of a conventional design, including a tuner, an intermediate frequency amplifier, and a detector of video, as well as baseband processing that provides hue and saturation control, brightness and contrast control, and array formation for the component (eg, RGB) that is to be displayed. The kinescope can be of a monochromatic shape, or it can be of the color type (direct view or projection). For color video applications, three of the kinescope booster systems will be needed, one for each cathode to be boosted. The high voltage power (eg, about 200 volts) for the operation of the amplifier 20 and the feedback of the regulator circuits 50 and 60, is provided by the high voltage supply terminal (HV) 20. The decoupling of the supply of High voltage (20) is provided by a decoupling network or a low pass filter comprising resistor R20 and capacitor C20. A low voltage supply terminal (L.V.) 21 provides a relatively low voltage (e.g., Approximately 12 volts) to polarize the input and cascade stages (transistors Q1-Q3) of the high voltage video driver amplifier 20. This supply input is also decoupled by means of an RC network comprising resistor R21 and to capacitor C21. The high voltage drive amplifier 20 comprises an input transistor Q2 connected to the common emitter NPN, connected in cascade with an output NPN Q3 transistor connected to the common base. A fixed base bias voltage is provided for the cascade output transistor Q3 via the low voltage decoupling network (R21, C21) (eg, + 12 volts). A lower potential is provided for the operation of the load resistor of the emitter R6 of the input transistor Q2 by a Zener diode regulator comprising the resistor R5 and the Zener diode CR1 coupled between the base of the transistor Q3 and the ground. Illustratively, the Zener voltage may be 5 or 6 volts, which establishes a direct current reference for the load resistor R6 of the cascaded input transistor, as well as a direct current reference for the detection amplifier of the automatic polarization of the kinescope 40. The emitter electrode of the input transistor Q2 is also coupled to ground by means of a network of high frequency peaks comprising the resistor R7 and the capacitor C2, which are coupled in series. The video input signal to be amplified, provided by the source 10, is applied to the base of the cascaded input transistor by means of an input step of the emitter follower comprising the PNP transistor Ql, which is connects in the collector thereof with the ground, and is coupled in its base with the video input terminal 12 by means of an input resistor R3. The emitter of the transistor Ql is coupled to the base of the transistor Q2 and to the low voltage supply 21 by means of a transmitting resistor R4. Additional high frequency peaks are provided by an additional peaking network comprising the series connected resistor R1 and the capacitor C1 coupled in parallel with the input resistor R3. The collector load for the cascade amplifier 20 is provided by the resistor R8, which is coupled from the high voltage supply 10 to the collector of the cascade output transistor Q3. A diode CR3 is interposed between the load resistor R8 and the collector of transistor Q3 to provide a small phase-shift voltage to reduce cross-distortion in the follow-on amplifier of the complementary emitter 30. In the operation of the cascaded amplifier 20, the gain of open cycle is directly proportional to the value of the load resistor R8, and inversely proportional to the impedance of the emitter network R6, C2 and R7, as described above. The open cycle gain, the bandwidth, and the oscillation index, are also a function of the capacitive load of the output of the amplifier 20 (ie, the capacitance presented to the collector of the transistor Q3). This is reduced, as explained in detail below, by the operation of the push-pull transistors of the follower amplifier of the complementary emitter 30 in constant values of the collector voltage to the emitter. The closed cycle gain, assuming that the open cycle gain is adequate, is directly proportional to the value of the feedback resistor R2, and inversely proportional to the impedance of the input network Rl, R3, and Cl. Considering now the details of the amplifier follower of the complementary emitter in counter phase 30, this amplifier includes a pair of complementary transistors Q4 and Q7 coupled in the base electrodes thereof with the output (collector of Q3) of the amplifier 20, and coupled in their emitters with an output terminal 15 by means of the respective emitter resistors R9 and R12. The output 15 of the follower 30 is coupled, as noted above, with the cathode 16, by means of a kinescope arc suppression network comprising the series connection of the inductor Ll and the resistor R15. The supply voltage (collector potentials) for the transistors of the follower Q4 and Q7 is provided by the respective feedback circuits 50 and 60.
The circuit 50 provides the function of regulating the collector voltage to the emitter of the follower transistor Q4 at a fixed value. For this purpose, the circuit 50 includes a voltage regulating transistor Q6 connected in its collector with the supply 20, and in its emitter with the collector of the transistor Q4. The input (base) of the voltage regulating transistor Q6 is coupled to the emitter electrode of the follower transistor Q4 by means of a capacitor C3 in parallel with a threshold conduction device (ie, a Zener diode) CR3. This positive feedback path establishes a collector offset voltage to the substantially constant emitter for follower transistor Q4 equal to the Zener voltage. To provide an operating current for the Zener diode, its cathode is coupled to the high voltage source 20 by means of a resistor Rll. To minimize the charge of the emitter circuit of transistor Q4, the emitter is coupled to capacitor C3 and Zener diode CR3 by means of a transistor follower of emitter Q5. Specifically, transistor Q5 is a PNP transistor coupled at its base with the emitter of follower transistor Q4 by means of a resistor RIO. The collector path to the emitter of the follower transistor Q5 is coupled between the junction of the capacitor C3 and the Zener diode CR3 and the ground. In certain applications, transistor Q5 may be omitted, as will be shown and will be described in a further example of the invention. The circuit 60 is similar to the circuit 60, and provides the function of regulating the collector voltage to the emitter of the follower transistor Q7 at a fixed value. For this purpose, the circuit 60 includes a voltage regulating transistor Q9 connected in its manifold with a supply input of the detection amplifier 40, and in its emitter with the collector of the transistor Q7. The input of the voltage regulating transistor Q9 is coupled to the emitting electrode of the follower transistor Q7 by means of a capacitor C4 in parallel with a threshold conduction device (ie, a Zener diode) CR4. This feedback path regulates the collector voltage to the emitter of the follower transistor Q7 at the Zener voltage. To provide an operating current for the Zener diode, its anode is coupled to the ground by means of a resistor R14. To minimize the load of the circuit the emitter of the transistor Q7, the emitter is coupled with the capacitor C4 and the Zener diode CR4 by means of a transistor follower of the emitter Q8. In a specific manner, the transistor Q8 is an NPN transistor coupled in its base with the emitter of the follower transistor Q7 by means of a resistor R13. The collector path to the emitter of transistor Q8 is coupled between the junction of capacitor C4 and Zener diode CR4 and high voltage supply 20. Detection amplifier 40 is provided for use in video deployment systems of the type providing circuitry Automatic polarization of the kinescope (AKB), and therefore, requires the detection of the cathode current of the kinescope "Ik". The amplifier 40 comprises a current detecting transistor of the cathode Q10 connected in its emitter with the collector of the voltage regulating transistor Q9. A reference potential is provided for the base of the transistor Q10 by the Zener diode CR1. The capacitor C5, in parallel with the diode CR1, provides filtering of the regulated Zener voltage. An output voltage, proportional to the cathode current Ik, is developed at the output terminal 18 through the load resistor R16 coupled between the collector of the transistor Q10 and the ground. In applications that do not require the automatic polarization operation of the kinescope, the detection amplifier can be omitted. If so, as shown in the last example, the collector of the voltage regulating transistor Q3 must be coupled to ground or to another suitable low-voltage reference potential. To summarize the operation described above, the cascade amplifier 20 amplifies the video signal provided by the source 10 as described above. To minimize the capacitive load on the load resistor R8 which can be attributed to the capacitance associated with the kinescope 16, its socket and spark arrestors (not shown), and other parasitic capacitances, the output (collector of transistor Q3) of the amplifier in cascade 20 is coupled to the kinescope cathode electrode by means of a complementary emitter follower amplifier in counter phase 30. This particular follower amplifier is of the "parallel" type, where the base electrodes are in parallel to receive the video signal amplified, and the emitters are in parallel to drive the cathode. The inclusion of the follower amplifier 30, as recognized herein, provides a reduction in the capacitance of the cathode presented to the amplifier 20, but introduces a secondary capacitance effect. That is, the collector capacitances to the base of the follower transistors Q4 and Q7. To effectively reduce the values of these unwanted capacitances, the reactive charge and discharge currents supplied to these capacitances are reduced. This characteristic is provided by the two positive feedback regulators 50 and 60 which maintain the collector voltages to the emitter for the follower transistors at constant values. As an example, if the output voltage of the amplifier 20 is increased, then the emitter voltage of the follower transistor Q4 will be increased, but the Zener diode CR3 and the regulating transistor Q6 will increase the collector voltage of the follower transistor Q4. In a similar manner, for a decreasing output voltage of the amplifier 20, the emitter voltage of the follower transistor Q4 will decrease, and the Zener diode CR3 and the regulating transistor Q6 will cause a decrease in the collector voltage of the follower transistor Q4. Illustratively, for a Zener voltage of 10 volts, the collector emitter voltage of transistor Q4 will be equal to the Zener voltage minus the junction voltages of the base to the emitter (Vbe) of transistors Q5 and Q6. For the assumed Zener voltage of 10 volts, the resulting voltage of the collector to the emitter of transistor Q4 in this manner will be approximately equal to 8.8 volts (assuming a Vbe value of 0.6 volts). Therefore, whether the follower input voltage is increasing or decreasing, the voltage across the follower transistor from the collector to the emitter is constant. As the input signal passes through the inflection points, the base voltage will vary by a few hundred millivolts relative to the emitter as the follower transistor is biased on and off (push-pull operation). However, it has been found that the voltage variations of the base emitter are relatively minor compared to the emitter voltage of the regulated collector (e.g., a Zener voltage of approximately 10 volts). As a result, it can be considered that the variations of the collector voltage to the base are "substantially" constant, and thus, there may be little charge and discharge of the capacitance of the collector to the base under dynamic signal conditions. Since these reactive currents are suppressed, according to the invention, the effective capacitances of the collector to the base for the follower amplifier are reduced. As described above, the feedback to regulate the collector emitter voltages for the follower transistors is almost 100 percent. It can never be exactly equal to 100 percent because the gain of transistors Q5 and Q6, for example, can not be equal to unity, since these would require infinite current gains. In other words, transistors Q5 and Q6 are both connected as emitter followers, and the gain of a follower of the emitter can be very close to the unit, but it will never be equal to the unit. In accordance with the above, even when the feedback is positive, the circuit is stable. Lesser amounts of feedback can be used, for example, 50 percent, if desired, in a given application. It will be noted that the actual Zener voltage is not a critical parameter of the circuit. The Zener bypass capacitor (C3 or C4) provides a desirable reduction in the alternating current impedance of the voltage regulator to further facilitate broadband operation. The example in Figure 1 can be modified as shown in Figure 2. In this example, the feedback control of the gain of the cascaded amplifier has been replaced by the forward power control, and the amplifier has been suppressed. automatic bias detection of kinescope 40. Additionally, voltage regulators 50 and 60 have been simplified. In more detail, in the high voltage cascade amplifier 20 of Figure 2, the feedback resistor R2 has been removed, as well as the input peak components of the resistor Rl and the capacitor Cl. The gain, thus modified, is determined by the load resistor R8 and the impedance of the emitter of the input transistor Q2 (ie, the emitter resistor R6 and the peaks comprising capacitor C2 and resistor R7). Apart from these modifications, the operation is otherwise the same as in the example of Figure 1. The omission of the kinescope 40 automatic polarization detection amplifier, as explained above, requires a relatively low potential source for the transistor collector positive feedback voltage regulator Q9. The collector could be connected to any suitable potential close to ground. Here, it is connected directly to the ground. The simplification of circuits 50A and 50B of the positive feedback voltage regulator comprises removing transistors Q5 and Q8, and removing resistors RIO and R13. In the above examples, these elements provide the coupling of the emitters of the follower transistors with the respective threshold conduction devices and capacitors. In this example, the emitter of the follower transistor Q4 is coupled to the capacitor C3 and the Zener diode CR3 by connecting these elements directly to the output terminal 15. The same is done for the capacitor C4 and the Zener diode CR4. In operation, the resistor Rll supplies current from the high-voltage supply 20 through the Zener diode CR3 to the output terminal 15. This establishes a regulated voltage at the base of the regulating transistor Q6, which is equal to the emitter voltage of the emitter. transistor Q4 minus the fall through resistor R9 plus the Zener voltage of diode CR3. Resistor R9 is provided primarily to provide protection against simultaneous conduction of transistors Q4 and Q7, and thus, may be of a relatively small value (eg, approximately 30 Ohms). In accordance with the foregoing, the voltage drop across the resistor R9 is negligible, and the transistor Q4 operates at a substantially constant collector emitter voltage. The operation of the modified feedback regulator 60A is the same as for the 50A, except for the polarities of the transistor and the directions of the current flow. Figure 3 illustrates a modification of the example of Figure 1, wherein the "parallel" shape of the follower of the complementary push-pull emitter 30 is replaced by a "in series" form of complementary push-pull emitter follower 30B. The modified follower comprises an NPN transistor Q302 having its base path to the emitter coupled in series with that of a PNP transistor Q306 between an input terminal 301 and an output terminal 308. The respective diodes CR300 and CR304 are coupled to each other. junctions of the base to the emitter of transistors Q302 and Q306, and polarize opposite to the polarization of the associated union. In accordance with the above, diode CR300 is conductive when transistor Q302 is biased to off, and vice versa. In a similar manner, the CR304 diode becomes conductive when the transistor Q306 is biased to off. The collector voltage to the emitter of transistor Q302 is adjusted to approximately the Zener voltage value of diode CR3 by connecting the RIO resistor to the emitter of follower transistor Q302 to detect the emitter voltage, and by connecting the transmitter of the transistor of the emitter. Q6 voltage regulator with collector of follower transistor Q302. This provides a positive feedback to regulate the collector voltage of transistor Q302 to an offset value of the emitter voltage and proportional to the Zener voltage of diode CR3. In a similar manner, the collector voltage to the emitter of transistor Q306 is regulated at approximately the Zener voltage value of diode CR4, by connecting resistor R13 to the emitter of follower transistor Q306, to detect the emitter voltage, and by the connection of the emitter of the voltage regulating transistor Q9 with the collector of the follower transistor Q306. This provides a positive feedback to regulate the collector voltage of transistor Q302 to an offset value of the emitter voltage, and proportional to the Zener voltage of diode CR4. Since the diode CR2 is not needed in the modified circuit, the load resistor R8 for the cascade amplifier 20 is connected directly to the collector of the cascade output transistor Q3, and this point is connected directly to the input 301 of the follower 30B. In operation, a rising video signal voltage at the input 301 will direct the bias transistor Q302 to supply the pulse current via the diode CR304 to the cathode of the kinescope 16, and the regulator 50 will maintain the emitter voltage of the collector of the transistor Q302 constant. A decreasing video signal voltage at the input 301 will direct the bias transistor Q306 to withdraw the impulse current by means of the diode CR300 of the cathode of the kinescope, and the regulator 60 will keep the collector emitter voltage of the transistor Q306 at a value substantially constant. For the purposes of detecting the automatic polarization of the kinescope, the collector current of the regulating transistor Q9 is applied to the detection amplifier circuit 40, whose operation is as described above. Figure 4 illustrates a modification of the example of Figure 1, wherein the "parallel" form of the follower of the complementary push-pull emitter 30 is replaced by a "in series" form of complementary push-pull emitter follower 30C. The modified follower comprises an NPN transistor Q400 having its base path to the emitter coupled in series with that of a PNP transistor Q402 between an input terminal 401 and an output terminal 409. The respective diodes CR404 and CR406 are coupled to each other. the junctions of the base to the emitter of the transistors Q400 and A402, and polarized opposite to the polarization of the associated junction. In accordance with the foregoing, diode CR404 is conductive when transistor Q400 is biased to off, and vice versa. In a similar manner, the diode CR406 becomes conductive when the transistor Q 02 is biased to off. The collector voltage to the emitter of transistor Q400 is adjusted to approximately the Zener voltage value of diode CR3 by connecting the emitter of transistor Q6 to the collector of transistor Q400, and by coupling the emitter of transistor Q6 to output 409 through capacitor C3 and the Zener CR3 diode. This provides a positive feedback to regulate the collector voltage of transistor Q400 to an offset value of the emitter voltage and proportional to the Zener voltage of diode CR3. In a similar way, the collector voltage to the emitter of transistor Q402 is regulated by approximately the Zener voltage value of diode CR4 by connecting the emitter of regulating transistor Q9 with the collector of transistor Q402, and by coupling the base of transistor Q9 with the output terminal 409 by means of capacitor C4 and Zener diode CR4. This provides a positive feedback to regulate the collector voltage of transistor Q302 to an offset value of the emitter voltage and proportional to the Zener voltage of diode CR4. Since the diode CR2 is not needed in the modified circuit of Figure 4, the charging resistor R8 for the cascade amplifier 20 is connected directly to the collector of the cascade output transistor Q3, and this point is connected directly to the entry 401 of the follower 30C. In operation, a rising video signal voltage at the input 401 will direct the bias transistor Q400 to supply pulse current via the CR406 diode to the kinescope cathode 16, and the regulator 50 will maintain the emitter voltage of the transistor collector Constant Q400. A decreasing video signal voltage at the input 401 will direct the bias transistor Q402 to withdraw the impulse current by means of the diode CR404 of the kinescope cathode, and the regulator 60 will maintain the collector emitter voltage of the transistor Q402 at a value substantially constant. Since the kinescope's automatic polarization detection is not required, the collector of the voltage regulating transistor Q9 is coupled with a relatively low voltage source (ground). Figure 5 illustrates a modification of the example of Figure 2 to provide an operation of the single-ended voltage follower. The term "voltage" follower, as used herein, refers to emitter trackers (which employ bipolar transistors) and trackers of a source (which employ field effect transistors). In this example of the invention, the voltage follower operates in a Class A mode, where the follower transistor is conductive at all times. This eliminates the cross-distortion that can occur in the complementary trackers where the transistors operate in a Class B mode with limited driving. On the other hand, Class B or push-pull operation is preferred from a power dissipation point of view, since efficiency is much higher than with single-ended trackers. As a brief overview, in this example of the invention, a video amplifier (20A) is coupled to a kinescope cathode electrode 16 by means of a voltage follower 500. The voltage follower comprises a transistor having a path of conduction and a control electrode to control the conduction of the trajectory. In this case, the voltage follower is an emitter follower, and the follower transistor is a bipolar transistor Q502. The control electrode (e.g., the base of transistor Q502) is coupled to receive a video signal from the video amplifier. A first end of this conduction path (e.g., the emitter of Q502) is coupled with a reference potential point (here, the ground) by means of a current source 504, and coupled with the kinescope cathode 16 The second end (ie, the emitter of Q502) of the conduction path is coupled with a source of supply voltage (20). A feedback circuit 50A couples with the first end of the conduction path to apply a positive feedback voltage to the second end of the conduction path of the voltage follower transistor (Q502) to maintain a substantially constant voltage across the path of conduction that is independent of the variations in the video signal applied to this control electrode. In more detail, in Figure 5, the output of the cascade amplifier 20A is coupled to the cathode of the kinescope 16 by means of a single-ended emitter follower amplifier 500 comprising a transmitter follower transistor Q502 connected on its base electrode to the output (collector) of the transistor Q3 on the cascade amplifier 20A. In this case, the load resistor of the collector R8 is connected directly to the collector of the output transistor Q3. The emitter of the transistor Q502 is coupled with an output terminal 506, which is coupled to ground by means of a current source 504 which provides a constant current pulse to the output terminal 506. The electrode of the cathode 16 is coupled with the output terminal 506 by means of the kinescope arc arrest network comprising the series connection of the resistor R15 and the inductor Ll. To regulate the collector voltage to the emitter of the transistor follower of the emitter at a constant value, the emitter is connected to the capacitor C3 and the Zener diode CR3 of the positive feedback voltage regulator circuit 50A. The output of this regulator is the emitter of the regulating transistor Q6, which is coupled with the collector electrode of the follower transistor of the emitter Q502. The operation of the emitter follower is similar to the operation of the corresponding transistors described above, except with respect to the efficiency and cross-effects described above, and to the method for providing a negative coupling current. Specifically, for the decreasing values of the video signal, the reduction of the cathode voltage is provided by the current source 504. Although this source may comprise a passive element such as a resistor, an active device may be preferable in certain applications, for example, where a faster negative oscillation index is desired at low output voltage levels. For this purpose, a constant current source, such as a suitable polarized bipolar or field effect transistor, is suitable. To reiterate the overall operation, when the amplified video signal provided by the amplifier 20A is increasing its voltage, the emitter voltage of the transistor Q2 will also be increased, thereby raising the base potential of the feedback regulating transistor Q6, and maintaining this way the collector voltage to the emitter of the transistor follower of the constant emitter. Since this voltage does not change in a significant way, there is no charge of the capacitance of the collector to the base of the transistor Q502, and thus, the effective capacitance presented before the output of the amplifier 20A is reduced over that of an amplifier follower. conventional emitter. Conversely, as the base voltage drops, so does the emitter voltage and transistor Q6, which is offset from the emitter voltage by the Zener voltage of diode CR3, and decreases the collector voltage of the follower transistor Q502 to maintain a constant voltage from the collector to the emitter. In this latter case, there is no active negative coupling of the video output voltage, but this function is provided by the current source 504. FIG. 6 illustrates a modification of the example of FIG. 1 with respect to the manner of providing a base pulse current for transistor Q8. In a specific manner, in Figure 1, the base of transistor Q8 was connected to the emitter of follower transistor Q7 by means of a resistor, while, in Figure 6, the base of transistor Q8 is coupled by means of a diode CR600 and a capacitor C6 with the emitter of transistor Q8, and coupled by means of a resistor R600 with the emitter of transistor Q5. The purpose of the above changes is to reduce detection errors of the cathode potential current (Ik) by direct-current biasing of the base of transistor Q8 from the emitter of transistor Q5. This eliminates the current demand of the direct current base for transistor Q8 from the emitter of transistor Q7, which conducts the cathode current Ik. The aggregate capacitor C6 provides an alternating current coupling of the emitter of the transistor Q7 with the base of the transistor Q8, and thus, the high frequency operation is the same as in the previous example. The added diode CR600 provides a correction for video signal conditions that involve high frequencies and high duty cycles. In a specific manner, this diode provides a direct current path around the alternating current coupling capacitor for the high duty cycle high frequency signal conditions, to prevent a reduction in base bias for the Q8 transistor under the conditions of High frequency signal and high duty cycle. Briefly, the CR600 diode prevents the capacitor C6 from developing a significant average load that would otherwise tend to reduce the base bias of the Q8 transistor for the high frequency and high duty cycle video signals. In greater detail, it has been discovered that under certain conditions in the example of Figure 1, the current demand due to the base current of transistor Q8 may introduce an undesired error in the measurement of cathode current Ik by the detection amplifier. 40. At the point where Ik is measured for purposes of automatic kinescope polarization, the cathode is close to the cut (a high voltage level), and consequently, the current through the base circuit of transistor Q8 is relatively high , and in this way can cause a significant error in the measurement of cathode current Ik. The modifications described above ensure that, for direct current and low frequencies, the base current of transistor Q8 comes from the emitter of transistor Q5, thereby reducing the measurement error of Ik. However, for an optimal high frequency response during the active video ranges, it is not desirable to only push the base of the transistor Q8 from the emitter of the transistor Q5. For this case, more impulse is required, that is, for the active video signals (video displayed in comparison with the video measurement levels in the kinescope's automatic polarization operation), the transistor Q8 must receive the benefits of the phase change low and high current counter phase providing the coupling to alternating current from the emitter of transistor Q7. The function of the aggregate diode CR600, in the base pulse circuit for transistor Q8, is to provide, for occasions when the impeller is subjected to a high duty cycle, high frequency and high amplitude signals which would otherwise give as a result of transistor Q8 changing its polarization point. For these transient conditions, the aggregate diode CR600 provides a shunt around the AC coupling capacitor C6. It will be seen that other different changes can be made to the examples of the invention shown and described herein. For example, the cascade amplifier 20 may be provided with an active collector load rather than the passive load (resistor) shown. A suitable active load would be a polarized transistor to operate as a current source. Another modification to the load impedance of the cascaded amplifier would be to couple an inductor in series with the resistor R8. Another alternative would be to couple a small capacitance from the output of the push-pull amplifier to a "central branch" on the load resistor R8 to optimize overall operation. To facilitate central shunt, resistor R8 can be formed from two smaller value resistors connected in series with the common connection used for the tap point.