JPS5936015Y2 - Speed modulation amplifier - Google Patents

Description

[Detailed description of the invention] The present invention provides a speed modulation amplification device that uses a simple configuration to prevent the output transistor from being destroyed due to the emitter-base reverse voltage of the output transistor exceeding its rating. This is what we are trying to provide.

Embodiments of the present invention will be described below, taking as an example an apparatus that performs contour compensation by modulating the horizontal scanning velocity of an electron beam of a television receiver (hereinafter referred to as velocity modulation).

First, FIG. 1a shows a pulsed signal as an example of a luminance signal, and contour compensation will be explained below according to this example.

The luminance signal is then differentiated to generate the waveform shown in FIG.

This signal is then used to modulate the horizontal scanning speed of the electron beam of the cathode ray tube as shown in FIG. 1C.

Specifically, a velocity modulation coil is installed in the neck of the cathode ray tube, separate from the main deflection coil, and a current amplified to the required level is applied to the differential signal shown in Figure 1 through this coil, thereby adjusting the horizontal direction of the electron beam. It modulates the scanning speed.

As a result, in the first half of the edge rise, the scanning speed becomes faster, so the screen becomes darker, and in the second half of the edge, the horizontal scanning speed becomes slower, so the screen becomes brighter.

Then, as shown in FIG. 1d, the horizontal contours are emphasized on the screen, and an image with improved apparent sharpness can be obtained.

FIG. 2 shows an example of the configuration of a device that performs contour compensation using such scanning speed modulation.

In the figure, 1 is a capacitor for differentiation, 2 and 3 are bias resistors, 4 is an amplification transistor, 5 is its collector resistance, 6 is a temperature compensation diode, 7 is an emitter resistance, and 8
is a bias resistor, and 9 is a bypass capacitor.

Further, 10.11 is a transistor operating as an emitter follower, and 12.13 is an emitter resistor.

14.15 is a coupling capacitor, 16.17 is the base resistance of the output transistor 18.19, respectively, 20, 21.
22, 23, and 24 are bias resistors, 25 are smoothing capacitors, 26, 27 are emitter resistors, 28 are coupling capacitors, 29 are velocity modulation coils, and 30 are braking resistors.

In this device, a luminance signal is differentiated by a capacitor 1 and amplified by a transistor 4.

Then, the emitter follower of the transistor 10.11 in the next stage drives the output transistor 18.19.

The output transistor has a class B push-pull configuration for efficiency.

In addition, in this configuration, unlike a general power amplifier, it is necessary to flow a current signal instead of applying a voltage signal to the load, so the speed modulation coil 29 is connected to the collector side to amplify the current. .

In addition, since the current flowing through the speed modulation coil 29 is the one obtained by differentiating and amplifying the video signal of television broadcasting, its frequency is very high (more than 3 MHz) and the current flowing through the speed modulation coil 2
The voltage induced in 9 becomes very large.

Therefore, it is necessary to reduce the inductance of the speed modulation coil 29 to suppress the induced power.

In order to create a magnetic field sufficient to modulate the electron beam scanning speed, as described above, the speed modulation coil 29
Since the inductance of the velocity modulation coil 2 becomes smaller,
It becomes necessary to increase the current flowing through 9.

For this purpose, it is necessary to apply a signal with a relatively large amplitude to the bases of the output transistors 18, 19.

However, when a large amplitude signal is applied to the bases of the transistors 18 and 19, the emitter-base voltage causes a drop in hFE, or in severe cases, the transistors are destroyed.

This situation will be explained using a sine wave as an example with reference to FIG.

When a relatively large amplitude signal is applied to the base of the transistor 18 as shown in FIG. causes a drop.

That is, the potential of the emitter is approximately the same as the potential of the base.

However, during the cut-off period of the transistor 18, the emitter current becomes zero, and the emitter potential does not rise above the power supply voltage terminal B2, but since the signal is still applied to the base, the base potential becomes higher than the emitter potential, exceeding the rated value. vEB is applied to transistor 18.

Figure 3 shows the emitter waveform. vEB in the figure is applied to transistor 18.

(Note that the vEB rating of a normal silicon transistor is about 5■.

) Of course, with regard to the transistor 19, a phenomenon in which B exceeds the rating also occurs during the cut-off period.

For this reason, in the past, a diode was connected in series between the emitter of the output transistor 18 and the emitter resistor 26 to avoid this phenomenon.

(Of course, a diode is also connected between the emitter of the transistor 19 and the emitter resistor 27.

) In this method, the diode is also cut off during the cut-off period of the transistor 18, and the vEB applied to the transistor is halved.

However, during the conduction period of the transistor 18, an emitter current flows through the diode, but as described above, this current has a frequency of up to 3 MHz, and is a considerably large current.

For this reason, the diode inserted into the emitter must have good frequency characteristics and withstand large currents, but diodes that satisfy such characteristics are extremely expensive.

The present invention eliminates the above-mentioned conventional drawbacks.

An example of this will be described below with reference to FIGS. 4 and 5.

FIG. 4 shows a configuration example.

In this embodiment, the capacitor 31 is replaced by the output transistor 18.
It is characterized in that it is coupled between the and 19 emitters.

In the figure, the same elements as in FIG. 2 are given the same numbers.

Output transistors 18 and 19 are connected to resistors 20 as in FIG.
．． It is biased to class B by 21°22 and 23, and the current is amplified by half waves.

That is, when transistor 18 is conductive, transistor 19 is turned off, and conversely, when transistor 18 is turned off, transistor 19 is turned on.

If a sine wave is applied to the bases of transistors 18 and 19, then during the conduction period of transistor 18, its emitter and base are at almost the same potential as explained in the conventional example.

On the other hand, during the cutoff period of transistor 18, transistor 1
One is conductive, and the emitter of the transistor 19 has the same potential as the base of the transistor 19, that is, a similar waveform is generated.

Here, Ko? Capacitor 31 connects transistors 18 and 19
Since the emitters of the transistor 18 are connected to each other, the emitter of the transistor 18 is connected to the emitter of the other transistor 1 even during the cut-off period.
The emitter waveform of 9 is added, and the potential becomes the same as that of the base.

This situation is shown in Figure 5 a, l). Similarly, during the cut-off period of the transistor 19, the emitter waveform of the transistor 18 is applied to the emitter of the transistor 19, and the emitter and base of the transistor 19 have approximately the same potential.

As a result, the voltage between the emitters and bases of the transistors 18 and 19 is small, and destruction of the transistors can be prevented.

Further, since the capacitor 31 must transmit a signal up to the low cutoff frequency of the amplifier to an emitter resistance of usually several ohms, its value must be sufficiently large.

Note that the emitter resistance needs to have a resistance value above a certain level in order to make the emitter and base almost at the same potential.

As described above, according to the present invention, by simply adding one capacitor, it is possible to reliably prevent destruction of the output transistor due to reverse voltage between the emitter and base of the output transistor, and it is possible to prevent damage to the output transistor due to reverse voltage between the emitter and base of the output transistor. at high frequency,
Moreover, it is extremely useful for circuits that flow large currents.

[Brief explanation of the drawing]

Figures 1a to d are diagrams for explaining the principle of contour compensation using velocity modulation, Figure 2 is a circuit diagram of a contour compensation device used for velocity modulation, and Figures 3a and l) are diagrams showing the output transistors in Figure 2. FIG. 4 is a circuit diagram of an amplifier device according to an embodiment of the present invention, and FIGS. 5A and 5L are waveform diagrams of various parts of an output transistor according to the present invention. 18, 19... Output transistor, 26.27
...Emitter resistance, 31...Condenser.

Claims (1)

[Scope of utility model registration request] The collectors of two transistors with different polarities are connected to each other, and this connection point is connected to a scanning speed modulation coil that performs contour correction by modulating the horizontal scanning speed of the electron beam of the cathode ray tube. The emitters of each of the transistors are connected to the power supply terminal through the respective resistors, and between the emitters of the two transistors,
A speed modulation amplifier characterized by connecting a capacitor between the base and emitter to prevent reverse voltage breakdown.