US2752433A

US2752433A - Negative feedback amplifiers

Info

Publication number: US2752433A
Application number: US185916A
Authority: US
Inventors: White Eric Lawrence Casling; Stillwell Peter Frederic Cryer
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1948-06-30
Filing date: 1950-09-21
Publication date: 1956-06-26
Anticipated expiration: 1973-06-26

Description

June 26, 1956 Q wHlTE ET AL 2,752,433

NEGATIVE FEEDBACK AMPLIFIERS Filed Sept. 21, 1950 Laventopa EL, C. lsfhz'il'e FTC G'bILLLUGLL United NEGATIVE FEEDBACK AMPLIFIERS Claims priority, application Great Britain September 26, 1949 3 Claims. (Cl. 179-171) This invention relates to negative feedback amplifiers.

In negative feedback amplifiers which are required to amplify a wide range of frequencies, such for example as amplifiers utilised in scanning circuits in television equipment, difficulty is sometimes experienced because the amplifier may include elements, such as transformers, which cause considerable phase shift of higher frequency components relative to lower frequency components in said range of frequencies. This may result in the feedback becoming positive for the higher frequency components thereby rendering the amplifier liable to instability at the frequencies on which the feedback becomes positive.

In the copending United States application Ser. No. 100,694, filed June 22, 1949, by Eric Lawrence Casling White, and which has issued as Patent No. 2,652,459 on September 15, 1953, there is described an improved amplifier construction with a view to reducing the difficulty referred to in the preceding paragraph and there is also described a thermionic valve having at least a cathode, a control electrode, and an output electrode, an input circuit for applying signals to be amplified to said control electrode, a transformer having a primary winding in circuit with said output electrode, an output circuit including a secondary winding of said transformer and a resistance in series with said secondary winding, filter means for applying signals set up across said resistance to aid input circuit to afford negative feedback to said input circuit, a resistance in the cathode lead of said valve, and filter means for applying signals set up across said second resistance to said input circuit to afford negative feedback to said input circuit, said first and second filter means having complementary frequency characteristics with the pass-band of the first means in a frequency range in which feedback signals from said output circuit have appropriate phase to afford negative feedback to said input circuit, and with the pass-band of the second means in a frequency range in which signals from said output circuit would afiord positive feedback if fed to said input circuit, and said first and second resistances being dimensioned to set up feedback signals of substantially equal amplitudes at least at the changeover frequencies of said filter means.

In the particular form of the invention illustrated in the aforesaid application the amplifier is employed in the frame scanning circuit of television equipment and comprises a valve 1 to the control electrode of which the scanning waveform is applied after amplification in a pair of cathode coupled valves 5 and 6. A transformer 2 in the anode circuit of the valve 1 feeds the scanning waveform to the scanning coils 4 in the secondary circuit of the transformer. A first feedback path is provided from the resistance 16 in the secondary circuit of a transformer 2, and it comprises a resistance 17 and condenser 18 and a second feedback path is provided from the cathode of the valve 1 via condenser 19, the

feedback signals for this path being set up across a resistance 20 in the cathode lead of the valve 1. The feedback signals from both paths are fed to the control electrode of the valve 6 in the appropriate phase.

The object of the present invention is to provide a modified form ofamplifier which displays characteristics similar to the amplifier described in the aforesaid application and which displays advantages in certain applications of such amplifiers.

According to the invention there is provided a negative feedback thermionic valve amplifier comprising an input circuit, an amplifying path leading from said input circuit and including a thermionic valve having an output electrode, a complex load impedance connected to said output electrode, filter means leading to said input circuit, means for applying voltage variations set up across said complex impedance to said filter means, said filter means being dimensioned to transmit voltage variations in a relatively low frequency range and to attenuate voltage variations in a higher frequency range, a resistance connected in a space current path of said valve, second filter means leading to said input circuit, and means for applying voltage variations set up across said resistance to said second filter means, said second filter means being dimensioned to transmit voltage variations in said higher frequency range and to attenuate voltage variations in said lower frequency range, and the respective filter means being connected to produce negative feedback to said input circuit in response to the transmitted voltage variations.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be more fully described with reference to the accompanying drawings in which:

Figure 1 illustrates one example of the present invention.

Figure 2 illustrates a modification of Figure 1.

Figure 3 illustrates another modification of Figure 1.

Referring to the drawing the amplifier illustrated in Figure l is embodied in a frame scanning circuit of television equipment and is of the same general construction as the amplifier illustrated in the aforesaid application, corresponding parts being indicated by the same reference numerals. It comprises a pair of cathodecoupled amplifying valves 5 and 6, the input signals being applied to the control electrode of the valve 5 and the valve 6 having a suitable anode load, the signals set up across which are fed to the control electrode of an output valve 1. This latter valve feeds the scanning coils 4 via an output transformer denoted in general by the reference character 2, the primary winding of the transformer being connected in the anode lead of the valve. The input signals consist of sawtooth waveform potentials of frame frequency and are applied to the control electrode of the valve 5 via a large resistance 23. Furthermore the transformer 2 has two

secondary windings

24 and 25, the scanning coils 4 being fed from the winding 24. The winding 25 is earthed at one end and coupled for feedback, via a large resistance 26, to the control electrode of the valve 5. A second feedback path is provided from a small feedback resistance 27 in the anode lead of the valve 1 to the control electrode of the valve 5, the anode end of the resistance 27 being coupled via a condenser 28 to a tapping 29 on the resistance 26 located above the aforesaid virtual earth point. The other end of the resistance is decoupled to earth via a condenser 30.

It will be appreciated that the load impedance presented to the anode of the output valve 1 is complex since the coils 4 and transformer 2 have a resistive component and a reactive component of impedance. Moreover, the

reactive component of the impedance will change with frequency, giving rise to phase displacements in the voltage variations set up across the impedance. In operation of the arrangement, the lower frequency components of the voltage variations set up across the secondary winding 25 have effectively the same phase as the current variations in the primary circuit of the transformer and hence the voltage variations set up across the feedback resistance 27, since at the lower frequencies encountered in a frame scanning circuit, the impedance of the transformer 2 in practical cases is mainly resistive. For lower frequency components feedback is received at the control electrode of the valve 5 mainly from the winding 25 via the resistance 26, the phase of such feedback being appropriate to provide negative feedback. At lower frequencies the impedance of the condenser 28 is so high as effectively to prevent feedback being received from the resistance 27. The higher frequency components of the voltage variations set up across the winding 25 are displaced in phase relatively to the corresponding current variations in the primary circuit of the transformer, and hence in relation to the higher frequency components set up across the resistance 27, and are not suitable for negative current feedback. However, at such frequencies the impedance of the condenser 28 is low and a low impedance path is provided from the tapping 29 to ground via the condenser 28, resistance 2'7 and decoupling condenser 30. Therefore the higher frequency components of voltage variations set up across the winding 25 are substantially attenuated at the tapping 29 and rendered ineffective to cause feedback to the control electrode of the value 5 and the liability to instability due to the phase shift produced in such components by the transformer 2 is substantially reduced. On the other hand, due to the low impedance of the condenser 28 at such frequencies the higher frequency components of the voltage variations set up across the resistance 27 are less attenuated and afford feedback to the control electrode of the valve 5 and since the transformer 2 is not efiective to produce a phase shift of these components, their phase is appropriate for providing negative feedback. Moreover, the tapping 29 is so located that the voltage variations received at the tapping 29 from the resistance 27 at the higher frequencies have the same amplitude as the corresponding voltage variations received at the tapping 29 from the winding 25 at lower frequencies, so that negative feedback, as if from a single feed point, is applied over a wide range of frequencies and the amplifier is maintained stable throughout a wider range of frequencies than would otherwise be the case. In effect, the feedback to the input circuit of the amplifier is derived from the winding 25 and the resistance 27 via filters having complementary frequency characteristics, that is to say the transmission characteristic of one of the feedback paths is the same as the attenuation characteristic of the other feedback path, this result being inherent in the direct connection of the high potential ends of the winding 25 and of the resistance 27 by means of the condenser 28 and the upper part of the resistance 26 in series, as shown in the drawing. Moreover, the pass-bands of the separate feedback paths are such that the feedback is derived from the winding 25 in the frequency range in which the voltage variations set up across the winding are not subjected to such phase shift with respect to currents in the primary winding as would give rise to oscillations, whilst for higher frequency components the feedback is derived from the resistance 27, the current in which is not subjected to such phase shift. In operation of the arrangement, a virtual earth point occurs near the lower end of the resistance 26, the junction point of the

resistances

23 and 26 being a point of small potential excursion and the valve operating with a low input impedance.

Instead of providing the resistance 27 in the anode lead of the valve 1, in series with the primary winding of the transformer 2, it may be provided in the lead to the screen electrode of the valve 1, in either case the resistance 27 being connected in a space current path of the valve 1, that is a path traversed by at least some fraction of the cathode current of the valve 1. In this case since the current traversing the resistance 27 will only be a fraction of that traversing the primary winding of the transformer, the magnitude of the resistance 27 will require to be increased. This modification is illustrated in Fig. 2. Moreover, in some cases the winding 25 of the transformer 2 may be dispensed with, and the resistance 26 connected directly to the lower end (in the drawing) of the primary winding of the transformer 2. This modification is illustrated in Figure 3.

What we claim is:

l. A negative feedback thermionic valve amplifier comprising an input circuit, an amplifying path leading from said input circuit and including a thermionic valve having an output electrode, a complex load impedance connected to said output electrode, filter means leading to said input circuit, means for applying voltage variations set up across said complex impedance to said filter means, said filter means being dimensioned to transmit voltage variations in a relatively low frequency range and to attenuate voltage variations in a higher frequency range, a resistance connected in a space current path of said valve, second filter means leading to said input circuit, and means for applying voltage variations set up across said resistance to said second filter means, said second filter means being dimensioned to transmit voltage variations in said higher frequency range and to attenuate voltage variations in said lower frequency range, and the respective filter means being connected to produce negative feedback to said input circuit in response to the transmitted voltage variations.

2. A negative feedback thermionic valve amplifier comprising an input circuit, an amplifying path leading from said input circuit and including a thermionic valve having an output electrode, an output circuit including a transformer having a primary winding connected to said output electrode, a feedback resistance connected in a space current path of said valve, a filter resistance connected from a winding of said transformer to said input circuit, and a filter condenser connected from said feedback resistance to an intermediate point on said filter resistance, said filter resistance and said filter condenser being dimensioned to transmit voltage variations from said winding in a relatively low frequency range, to attenuate voltage variations from said winding in a higher frequency range and to produce complementary transmission and attenuation of voltage variations from said feedback resistance, and said filter resistance and filter condenser being connected to produce negative feedback to said input circuit in response to the transmitted voltage variations.

3. A negative feedback amplifier comprising an amplifying path including a thermionic valve having an output electrode, a series feed resistance leading to said path for feeding signals to be amplified to said path, a compleX load impedance fed from said output electrode, a feedback resistance of low impedance compared with said complex impedance connected in a space current path of said valve, a filter resistance connected from said complex impedance to the end of said feed resistance nearer said path, and a filter condenser connected from said feedback resistance to an intermediate point in said filter resistance, said filter resistance and said filter condenser being dimensioned to transmit voltage variations from said complex impedance in a relatively low frequency range, to attenuate voltage variations from said complex impedance in higher frequency range and to produce complementary transmission and attenuation of the voltage variations from said feedback resistance, and said filter resistance and said filter condenser being connected to produce negative feedback to said input circuit in response to the transmitted voltage variations.

References Cited in the file of this patent UNITED STATES PATENTS Worcester June 17, 1941 10 6 Levy Dec. 24, 1946 Beurtheret July 28, 1953 FOREIGN PATENTS France May 16, 1938 France Dec. 7, 1938 France Apr. 4, 1945 France Sept. 4, 1945 Great Britain Nov. 19, 1941 Great Britain Sept. 14. 1943