US2173238A - Impedance coupled amplifier - Google Patents

Description

A. C. STOCKER IMPEDANCE COUPLED AMPLIFIER seplr. 19,1939.

Filed Dec. $1-, 1936 1 :Loo FREQUENCY 6 a. & 0 4

FREQUENCY Patented Sept. 19, 1939 UNETED STATES ATENT OFFICE IMZPEDANCE COUPLED' AMPLIFIER Arthur C. Stocker, Haddon Heights, N. J., assignor to Radio Corporation of America, a corporation of Delaware The present invention relates to resistancecapacity or impedance coupled amplifier for amplifying signals in a relatively wide frequency band.

The invention relates more particularly to resistance-capacity coupling networks for ampliers, as shown in my United States Patent 2,045,316, wherein the signal may vary in frequency between relatively wide frequency limits, such, for example, as from 20 cycles or lower to one megacycle.

In the system referred to, a resistance-capacity coupling network may comprise an output anode circuit for an amplifier tube comprising two branches, one of which includes an output anode coupling resistor and a lter capacitor in series relation to each other and to the anode circuit, and the other of said branches including a coupling capacitor and a load or grid resistor in series relation to each other, the said coupling elements and lter elements having a predetermined relation for improving the low frequency response of the amplifier provided with such network, and in accordance with the equation CgRg=RpCf where Cg=capacity of the coupling capacitor Rg=1oad resistance Rp=resistance of the anode coupling resistor Cf=capacity of the filter capacitor The operation of a circuit of that character depends upon loss of gain due to the reactance of the coupling capacitor becoming comparable to the resistance of the grid or load resistor, which is compensated by an equal rise in gain due to the reactance of the filter capacitor becoming comparable to the resistance of the anode circuit resistor.

Since a bypass or filter capacitor and filter resistor is employed in wide range amplifiers to reduce the liability of oscillation because ofr coupling in the common impedance of the anode voltage supply source, it is desirable to so choose the values of the iilter capacitor and resistance, that by means of compensation, a relatively low capacity value may be provided in the coupling capacitor. Because of the improved low frequency response in an audio frequency amplifier, the gain may be relatively high below 10 cycles per second, and this gives rise to undesirable response in the amplifier, known as breathing It is, therefore, an object of the present invention, to lower the gain, below a certain predeter.

mined critical frequency in an amplifier interstage resistance-capacity coupling network, without impairing the gain above the predetermined frequency.

Heretofore if an attempt is made to lower the 5 low frequency gain below the predetermined limit or critical frequency, by increasing the value of the bypass or filter capacitor, or by lowering the capacity of the coupling capacitor in one or more stages of an amplifier, the gain becomes too low at and above the critical frequency.

It is also a further object of the present invention to provide an improved interstage resistance-capacity coupling network providing a relatively high amplier gain at frequencies within an extended frequency range which may have .a response characteristic below a predetermined frequency gradually falling while above the predetermined frequency it may have a response characteristic which may remain substantially flat or otherwise as desired.

In applying compensation devices to a resistance-capacity network as an interstage coupling means for an audio frequency amplifier, it has been found that advantage may be taken of the anode circuit filter comprising a series impedance o-r resistor and a bypass circuit, the compensation `being obtained in the bypass circuit.

Accordingly, it is an object of the present invention to provide .an interstage coupling network of the resistance-capacity type, having a lter in the input or anode coupling portion thereof provided with a bypass circuit, the capacitive reactance of which is relatively low below a certain predetermined low frequency, and the capacitive reactance of which above such predetermined frequency is relatively high. In an audio frequency amplifier, this low frequency may be in the range below 100 cycles per second.

The invention will, however, be better understood from the following description when considered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing,

Figure 1 is a schematic circuit diagram of an audio frequency amplier stage provided with a resistance-capacity coupling network embodying the invention, and

Figures`2 and 3 are curves illustrating the operation of the circuit of Fig. 1, the curves of Fig. 2 being audio frequency response curves and the curves of Fig. 3 being reactance curves for certain circuit elements of Fig. 1.

Referring to Fig. 1, an audio frequency amplifying channel is shown, wherein an amplifier tube 5 is impedance coupled to a second stage amplifier tube 6, signals being applied to the first stage amplifier 5 through a grid input circuit I--8, and being derived from the second stage 6, through the usual anode output circuit indicated at 8.

The impedance coupling system is of the resistance-capacity type, comprising an anode output resistor I0, connected in the anode output circuit II of the first stage amplifier device 5, and being connected with a positive anode supply lead I2 Y through a series filter resistor I3. The negative anode potential supply lead may be 'assumed to be the chassis or ground indicated at I4, for example.

A bypass circuit, forming with the series filter resistor I3, a portion of the filter means in the anode circuit, is inclicated1byjthe shunt tuned circuit I6 and the series bypass capacitor II. The shunt tuned circuit I 8 comprises aninductance element I8 and a shunt tuning capacitor I9. The circuit is connected with a tap connection 20 between the anode output coupling resistor I8 and the series anode circuit lter resistor I3. The bypass circuit also includes, between the tuned circuit I6 and -ground I4, the serially connected bypass capacitor I'I.

The second stage amplifier grid circuit indicated at 2I includes a grid coupling impedance or resistor 22. The latter is connected to a bias potential supply lead 23 through a suitable series filter resistor 24 and the grid circuit is bypassed to the cathode lead indicated at 25, through a bypass or filter capacitor which provides a relatively low impedance path to signals in the frequency range of the amplifier, between the grid resistor 22 and the cathode.

The coupling between the grid circuit 2| and the anode output circuit II is provided by the coupling capacitor indicated at 2'I which, preferably, is of relatively low capacity. For relatively low frequency response, the product of the capacity of the coupling capacitor 2T and the resistance of the grid resistor 22 should be made substantially equal to the product of the resistance of the output coupling resistor I0 and the effective capacity of the filter elements I'I, I8 and I9 to provide relatively broad frequency response in accordance with the circuits shown in the patent referred to hereinbefore.

Referring now to Fig. 2 along with Fig. 1, if the low limit of the frequency range is extended down to a region below I0 cycles, for example, as indicated by the curve 30, the amplifier may breathe or provide low frequency bobbing or other undesirable response. However, it is desirable to provide a circuit having the substantially flat response curve similar to the curve 30 above a certain low frequency limit such as 30 cycles, for example, while, at the same time, having a relatively low gain below that predetermined frequency limit.

If an attempt is made tolower the low frequency gain below 30 cycles by lowering the reactance of the bypass circuit, as by the use of asingle large bypass capacitor at II in place of the circuit shown, or by decreasing the capacity of the coupling capacitor 21, the curve indicated at 32 is found to result, indicating a relatively poor response characteristic `above the predetermined limit of 30 cycles.

It has been found that the response characteristic of the curve 30 above the present arbitrary limit of 30 cycles, may be obtained while providing the response characteristic of the curve 32 below 30 cycles, by introducing a parallel tuned circuit such as that shown at I6 in the filter bypass circuit. The action of this circuit may best be understood by examining the reactance diagrams of certain bypass circuit elements, the curves of which are indicated in Fig. 3.

In Fig. 3, the curve 34 represents the reactance of a large by-pass capacitor for the anode circuit filter which would give a response curve in the amplifier of Fig. 1 along the curve 32 and the curve 36 is the reactance of the relatively Vsmall by-pass capacity necessary to raise the arnplifier response from that of the curve 32 of Fig. 2 to that of the curve 30. The curve 38-38 indicates the reactance of the by-pass circuit of Fig. 1, elements I'I, I8 and I9, and it will be seen that from approximately the dotted line A, up in frequency, the curve 39 approximates the curve 35 and that from approximately the line B down in frequency, the reactance indicated by the curve 38 is substantially equal to or smaller than the reactance indicated by the curve 34. Thus, the requirements are fulfilled for these two regions, although the peak between the two limits A and B may reach a very high reactance value, but the effect is limited by the resistance of the filter resistor I3. This reactance curve between the curves 38 and 39 is indicated at 40.

Using the circuit of Fig. l, the response curve 42 of Fig. 2 was obtained in an impedance coupling network, the constants of which are the same as for obtaining the curve 32 of Fig. 2, but with the addition that the inductance I8 equals 26 henries with a direct current resistance of 860 ohms, with a capacitor I 8, of 2.25 microfarads, and with the series filter resistor I3 reduced to 5,000 ohms.

It will be seen that with this circuit the coupling capacitor 21 may be reduced While the bypass capacitor I7 may be increased, without obtaining the undesirable effects of these changes above a predetermined frequency such as 30 cycles, for example, in the range below cycles, while breathing and undesired response is cutoif rapidly below that frequency.

Further referring to the reactance curves vof Fig. 3, the reactance of a by-pass capacitor alone in a filter network forming part of a resistancecapacity coupling means or network may be represented by the curve 34, while the reactance of the tuned circuit I6 may be represented by the curves 46, and the combined reactance of the Vseries filter capacitor and the tuned circuit is represented by the curves 38-39-40- Thus, there is substituted for the filter capaci- Yof a small capacitor above the frequency of resonance and the apparent capacitive reactance of which approximates the capacitive reactance of a large by-pass capacitor, below the frequency of resonance. It is thus possible to shift from this response curve 30 obtained with a small by-pass capacitor to the response curve 32 obtained with a large by-pass capacitor by decreasing the reactance through the frequency of resonance.

I claim as my invention:

1. In an impedance coupling network for an electric signal amplifier, the combination of a coupling resistor, a filter resistor in series with said coupling resistor, and a bypass circuit for said filter resistor connected between the junction of said resistors and ground, said circuit comprising a series capacity element, an inductance element and a shunt tuning capacity means for said inductance element, said vcircuit elements providing reactance in the bypass circuit approximating that of a relatively small bypass capacitor above a predetermined frequency and the reactance of a relatively large bypass capacitor below a predetermined frequency. y

2. An impedance coupling network in accordance with claim 1, further characterized by the fact that the inductance element and shunt tuning capacity means provide a shunt tuned circuit in series with said by-pass circuit responsive to a frequency below a predetermined limiting frequency in the response characteristic of said net- Work.

3. In an audio frequency amplifier, the combination with a pair of amplifier tubes, of a resistance-capacity network providing impedance coupling means between said tubes, said network including an anode output circuit for one of said tubes, a coupling resistor in said circuit, a filter resistor in series with said coupling resistor, and a bypass circuit for said filter resistor connected between the junction of said resistors and ground, said circuitcomprisinga series capacity element,.an inductance element and a shunt tuning capacity means for said inductance element, said circuit elements providing reactance in the bypass circuit approximating that of a relatively small bypass capacitor above a predetermined frequency and the reactance of a relatively large bypass capacitor below a predetermined frequency.

4. In a resistance-capacity interstage coupling network for signal amplifying apparatus and the like, means providing a relatively high uniform frequency response characteristic therein over a wide frequency band, a filter in said network having a series element and a shunt by-pass circuit including a filter capacitor, and means providing a tuned circuit in series with said capacitor, the combined capacitive reactance of the shunt circuit being relatively high above resonance of said tuned circuit to provide a high gain above a pre'- determined frequency, and the capacitive reactance of the shunt circuit below resonance of said tuned circuit being of a relatively low value suihcient to appreciably attenuate signals in a frequency range below said predetermined frequency.

5. In an audio frequency amplifier, the combination of an interstage resistance-capacity coupling network having a coupling resistor in one leg of said network, a series filter element for said leg of the network, and a by-pass circuit connected between said filter element and said coupling resistor, said by-pass circuit including a by-pass capacitor and a parallel tuned circuit in series therewith, the capacitive reactance oi said circuit being relatively low below a certain predetermined low frequency and being relatively high above said predetermined frequency.

6. A resistance-capacity coupling network including in combination, an output anode circuit comprising branch circuits, `one of said branch circuits including an output anode coupling resistor and a filter network in series relation to each other and to the anode circuit, said network including a shunt tuned circuit and a series capacity element, and the other of said branch circuits including a coupling condenser and a load or grid resistor in` series relation to each other, the product of the capacity of the coupling condenser and the resistance of the load resistor being substantially equal to the product of the resistance of the rst named output coupling resistor and the capacity of the filter network at frequencies above a critical frequency, and the product of the capacity of the coupling condenser and the resistance of the load resistor being substantially less than the product of the resistance of the first named output coupling resistor and the effective capacity of the filter network below the critical frequency.

7. In an electric discharge tube amplifier, a resistance-capacity interstage coupling network, comprising in combination, an output anode circuit, an output anode coupling resistor and a filter impedance element in series relation to each other in said circuit, a branch circuit included in` said output anode circuit comprising a coupling capacitor and a load resistor in series relation to each other, said coupling and branch circuit elements having a predetermined relation providing a relatively high gain throughout Va predetermined frequency response band above a predetermined critical frequency, and a bypass circuit for said filter impedance element comprising a shunt tuned circuit and a series capacity, said circuit having a capacitive reactance for causing a voltage generated by an amplifier device connected with said output circuit to be out of phase with the voltage across said output coupling resistor at frequencies below said critical frequency.

8. In a resistance-capacity amplifier couplingv network, a coupling impedance, a series filter element therefor and a bypass circuit for said series filter element comprising a shunt tuned circuit and a series capacity, said circuit elements providing a reactive network having an apparent capacity approximating a capacity value for causing said network to respond in a relatively low frequency range with high gain above a predetermined critical frequency and having an apparent capacity which is greater than said first named capacity in a frequency range below said critical frequency.

9. In an electric signal amplifier, an interstage coupling network comprising an anode circuit impedance element, a filter circuit in series therewith comprising a shunt tuned inductance and a series capacity element, a coupling capacitor and a load impedance connected in series and forming a branch circuit substantially in parallel with said series connected anode impedance and filter circuit, said coupling elements thereby forming two substantially parallel branch circuits having impedance values such that the product of the resistance of the anode impedance element and the apparent capacity of the filter bypass circuit at frequencies below the resonance frequency of the tuned inductance is materially greater than the product of the resistance ofthe load impedance and the capacity of the coupling capacitor and the product of the. resistance of the anode impedance element and the apparent capacity of the lter bypass circuit at frequencies above the resonance frequency of the tuned inductance is substantially equal to the product of the resistance of the load impedance and the capacity of the coupling capacitor at frequencies above the resonance frequency of said tuned inductance element.

10. In a resistance-capacity coupled amplifier, the combination of an output coupling impedance element, a filter element in series therewith, and means providing a bypass circuit for said filter element comprising a capacitor element and a tuned inductance element in series relation to each other, the reactance of said elements being such that the effective shunt capacity of said cir- 75 cuit increases in response to and attenuates signals below a predetermined critical frequency and that the effective shunt capacity effect of said circuit decreases to a predetermined value for signals in a frequency range above said critical frequency.

11. In an electric signal amplifier, a coupling network comprising resistance-capacity elements and including a coupling impedance across which signals are conveyed, a series filter element therefor, and means providing a shunt bypass circuit for said filter element, the capacitive reactance of which changes with frequency and which comprises a shunt tuned circuit, and a series capacitor providing a capacitive reactance of a predetermined value for signal frequencies above a predetermined critical frequency and having a relatively loW capacitive reactance below said critical frequency.

l2. A resistance-capacity coupled amplifier including an electric discharge amplifier device and an output coupling network therefor of the resistance-capacity coupling type, characterized by the fact that the coupling elements including a plate resistor Rp, an output coupling resistor Rg, a coupling condenser Cg, and a tuned filter network comprising a shunt tuned circuit and a series capacity of effective overall capacity C1 are -of such related impedance and capacity values that the equation CgRg=RpCf is substantially satisfied above a critical frequency, and that the expression CgRg RpCf, is satisfied below that critical frequency, whereby the output voltage across the output coupling resistor is substantially in phase with the voltage generated by said amplifier device at frequencies above the critical frequency and the voltage across the output coupling resistor is out of phase with the voltage generated by said device at frequencies below the critical frequency.

ARTHUR C. STOCKER.

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