Amplifier Circuit Having Controllable Gain

This invention relates to amplifier circuits having electrically controllable voltage gain and, in particular, to such amplifiers having stabilized, direct-current output potentials making them susceptible to direct coupling to subsequent stages.

Adjustment of amplifier gain is achieved by the variation of the effective value of at least one circuit element which is determinative of gain, for example, the transconductance of a transistor Conventional techniques for varying effective component values include transistors connected in parallel with a fixed resistor to vary the effective resistance thereof, or a voltage divider having an active element and a resistor to obtain a division ratio responsive to the conduction characteristics of the active element. Such variable networks, conventionally employed in the emitter, base, or collector circuits of transistor amplifiers, tend to disturb the quiescent bias of the amplier as well as varying its gain.

Alternatively, the bias condition of an active amplifier element may be varied in response to a gain control signal so that a gain influencing parameter of that transistor may be correspondingly varied. Such techniques conventionally give rise to wide fluctuations in the quiescent bias conditions of the amplifier necessitating alternating-current coupling to subsequent circuits.

The amplifier circuit of the present invention comprises a transistor stage, including a transistor and a load means, an amplifier in cascade connection therewith and current conductive means for varying the main current flow of the transistor. A degenerative feedback connection couples the output of the amplifier circuit to its input for controlling its quiescent operating point, thereby overcoming the disadvantages experienced with conventional gain-controlled amplifier circuits.

IN THE DRAWINGS:

FIGS. 1, 2 3 and 4 are schematic diagrams of amplifier circuits embodying the invention.

In FIG. 1, common-emitter amplifier transistor T.sub.1 receives input signals from terminal 10 at its base electrode through capacitor C. Load resistor R.sub.1 connects the collector of T.sub.1 to a relatively positive supply voltage at terminal 12. Resistor R.sub.3 supplies quiescent bias current to the base of T.sub.1 as will be explained later.

The voltage gain of the common-emitter amplifier stage is given by

where:

g.sub.m =transconductance of T.sub.1,

q=charge of an electron,

k=Boltzmann's constant,

T=absolute temperature (degrees Kelvin),

I.sub.Q =the value of quiescent current flowing in the collector-emitter conduction path of T.sub.1, and

R.sub.1 =the collector load resistance of T.sub.1.

The equation shows that the gain is directly proportional to the quiescent collector-to-emitter current flowing in T.sub.1.

Control transistor T.sub.2 conducts a portion of the current flowing through load means R.sub.1 through its collector-to-emitter conduction path in response to a control signal applied to its base via terminal 14. Application of a control signal tending to increase the conduction of current by T.sub.2 from node 18 causes a corresponding decrease in the collector-to-emitter current of T.sub.1, thereby effecting a reduction in the voltage gain exhibited by T.sub.1. Conversely, a control signal tending to decrease conduction in T.sub.2 increases the voltage gain of T.sub.1. Thus, transistor T.sub.2 provides current conductive means for controlling the voltage gain of the common-emitter amplifier stage.

Transistor R.sub.3 is an emitter-follower amplifier stage having an input connection at its base to which the collector of transistor T.sub.1 connects, and an output connection at its emitter which connects to output 16. The emitter of T.sub.3 connects through a current conductive connection, shown by way of example as resistor R.sub.2, to a further point of operating potential, shown by way of example as the reference potential, ground.

Resistor R.sub.3 completes a degenerative feedback connection between the non-inverting amplifier output 16 and the common-emitter amplifier input connection at the base of T.sub.1. This connection provides voltage feedback for biasing transistors T.sub.1 and T.sub.3 for conduction and for stabilizing quiescent bias potential as follows: quiescent potential at the base of T.sub.1 is established relative to the reference potential (ground) by the base-emitter forward conduction drop of T.sub.1. That potential also exists at output 16 because the potential across resistor R.sub.3, due to the base current demand of T.sub.1, is small as compared to the base-emitter conduction drop of T.sub.1. Similarly, the potential at the collector of T.sub.1 is offset from the potential at output 16 towards the potential at supply terminal 12 by the emitter-to-base offset potential of T.sub.3.

Operation of the degenerative feedback loop for stabilizing the quiescent potential at output 16 is as follows. Assume that the value of the control signal applied at terminal 14 is increased causing T.sub.2 to conduct additional current away from node 18. Assuming the current in T.sub.1 to be substantially independent of the control signal directly, the total current drawn through R.sub.1 thus tends to increase causing the potential drop across R.sub.1 to increase, in turn tending to reduce the potential at the base of T.sub.3. That tendency towards decreasing potential is coupled through the base-emitter of T.sub.3 to output 16 and thence through resistor R.sub.3 to the base of T.sub.1. Decreasing the base-emitter potential of T.sub.1 causes a corresponding tendency for decrease in its collector current tending to permit the potential at its collector to rise towards the potential at supply 12. That tendency is opposite in polarity sense to the tendency created directly by the control signal operating through T.sub.2. At balance, the decrease in T.sub.1 collector current is substantially equal to the increase in T.sub.2 collector current therfore the collector potential of T.sub.1 and the quiescent potential at output 16 correspondingly maintain substantially their original quiescent values. The response of the degenerative feedback loop to a decreasing control signal is similar to that with an increasing control signal except that the polarity senses of the responses are reversed.

Thus, transistors T.sub.1 and T.sub.2 taken together form a common-emitter amplifier whose gain may be varied by an electrical control signal, while transistors T.sub.1 and T.sub.3 taken together form an amplifier whose output quiescent potential is voltage stabilized by the operation of a degenerative voltage feedback connection. The circuit in toto comprises an amplifier having voltage gain responsive to an electrical control signal and whose quiescent output potential is substantally independent of that gain control signal. Such amplifiers are particularly advantageous because gain is adjustable over a wide range with substantially no variation of the quiescent bias voltages thereby providing a more efficient use of transistors to reduce cost and complexity, and increase reliability.

In FIG. 2, common-emitter amplifier transistor T.sub.1 and current-conductive control transistor T.sub.2 function substantially as described above for FIG. 1. The collector load for T.sub.1 is shown, by way of example, as current generator IS. Resistor R.sub.6 in the emitter circuit of T.sub.1 improves gain control linearity, restricts the control range, and tends to limit the growth of noise signals in the circuit under high-gain conditions.

The control signal at terminal 14 is applied to the base of transistor T.sub.2 in the voltage gain control circuit by way of a diode, shown by way of example as diode-connected transistor T.sub.6. T.sub.6 and T.sub.2 thus connected form a current-mirror amplifier susceptible to receiving control signals in the form of control currents.

The non-inverting amplifier of FIG. 2 comprises transistors T.sub.4 and T.sub.5 connected to form a push-pull amplifier in cascade connection after emitter-follower transistor T.sub.3. Signals from the collector of T.sub.1 are applied to the input of the non-inverting amplifier at the base of T.sub.3 which operates as an emitter-follower driving the base of T.sub.4. The emitter of T.sub.3 is returned to a relatively negative operating potential at supply terminal 20 by current supply means including resistor R.sub.2 and diodes D.sub.1 and D.sub.2.

Complementary transistors T.sub.4 and T.sub.5 form a push-pull amplifier having input connections at their respective bases and an output connection 16 at the common connection of their emitters. Diodes D.sub.1 and D.sub.2 provide an offsetting potential of like polarity to the forward conduction potentials of T.sub.4 and T.sub.5 tending to establish conduction therein. The collector of T.sub.4 connects to a relatively positive supply potential at terminal 12, shown by way of example through resistor R.sub.4. Similarly, the collector of T.sub.5 connects to relatively negative supply terminal 20 through resistor R.sub.5. The voltage feedback connection between output 16 and the base of T.sub.1 is provided by a current conductive connection, for example, resistor R.sub.3.

The operation of the amplifier of FIG. 2 is substantially similar to that of the amplifier of FIG. 1 as is the operation of the degenerative voltage feedback connection for stabilizing quiescent bias potentials. In a typical circuit, the value of R.sub.6 is preselected so that the voltage drop thereacross is small as compared to the base-emitter potential of T.sub.1. The quiescent potential at output 16 is substantially equal to the base potential of T.sub.1 rendering the amplifier susceptible for direct current coupling to subsequent circuits. The quiescent potential at the collector of T.sub.1 in the circuit of FIG. 2 however, is at a potential offset in the direction of the voltage at supply 12 by the base-emitter potentials of transistors T.sub.3 and T.sub.4.

Thus, where the forward-bias conduction potential of the base-emitter junction of a transistor is represented by V.sub.BE, output 16 has a quiescent operating potential of 1V.sub.BE relative to the reference potential (ground) in the circuits of both FIGS. 1 and 2, while the collector of T.sub.1 has a quiescent potential of 2V.sub.BE in FIG. 1, and the collector of T.sub.1 has a quiescent potential of 3V.sub.BE in the circuit of FIG. 2.

The amplifier of FIG. 3 is similar to that of FIG. 1 except that the order in which amplifier stages including T.sub.1 and T.sub.3 are cascaded is reversed. In operation, input signals from terminal 10 couple to emitter-follower amplifier stage T.sub.3 and thence to common-emitter stage T.sub.1. Output signals are available at terminal 16 from the collector of T.sub.1.

Control transistor T.sub.2 connects to node 18 for conducting, in response to the signal at 14, a portion of the current in load R.sub.1 that flows through the main-conduction path of T.sub.1. The emitter of T.sub.2 connects, by way of example, to relatively negative supply 20.

Direct-coupled degenerative feedback through R.sub.3 couples the output of amplifier stage T.sub.1 to the input of stage T.sub.3, at its base. Operation of that feedback loop is analogous to the description thereof for FIG. 1 hereinabove. Output 16 is controlled thereby to a quiescent potential of 2V.sub.BE.

FIG. 4 is a further alternative embodiment of the present invention wherein the first stage includes NPN transistor T.sub.3 in common-base configuration and the cascade stage includes PNP transistor T.sub.1 ' in common-emitter configuration.

In operation, input signals from terminal 10 couple to the emitter input to T.sub.3 and to load means R.sub.2 connected in its main-conduction path. Signals therefrom coupled to common-emitter transistor T.sub.1 ' stage and thence to output 16. Load means R.sub.1 couples the main-conduction path of T.sub.1 ' to relatively negative supply 20.

PNP control transistor RT.sub.2 ' controls the gain of the T.sub.1 ' stage in response to signals at terminal 14 by controlling the portion of the current flow through load means R.sub.1 that flows in the collector-emitter path of T.sub.1 '. T.sub.2 ' is shown, by way of example, as connecting to relatively positive supply terminal 13.

Degenerative feedback for stabilizing the output voltage at 16 is direct-coupled from the output of amplifier stage T.sub.1 ' at node 18 to the input amplifier stage at the emitter of T.sub.3 by resistor R.sub.3. Operation of that feedback loop is as follows. For example, increasing T.sub.2 ' collector current tends to increase the potential at node 18 which is coupled via R.sub.3 to tend to reduce the base-emitter potential of T.sub.3. That tendency reduces its collector current, increasing the potential at node 19, tending to reduce the base-emitter potential of T.sub.1 ' and the collector-emitter current therethrough. At balance, the reduction of current in T.sub.1 ' is substantially equal to the current increase in T.sub.2 ' therefore the potential at node 18 tends to maintain its original quiescent value (i.e. -1V.sub.BE in FIG. 4).

While the description and figures herein describe preferred alternative embodiments incorporating the present invention, one skilled in the art of design when armed with the teachings of this disclosure would be able to envision further embodiments without departing from the spirit of the invention. Control transistor T.sub.2 may be connected for supplying current so that the main current flow in T.sub.1 is greater than the current flow in load means R.sub.1 ; for example, in FIGS. 1, 2 or 3 T.sub.2 could be a PNP transistor having its collector connecting to node 18 and its emitter connecting to supply 12.

For example, a control transistor could be connected at its collector to node 19 in FIG. 4 for controlling the quiescent collector current of common-base transistor T.sub.3 and thereby controlling its gain. Alternative types of transistors may be substituted for the bipolar transistors shown; for example, control transistor T.sub.2 could be a field-effect transistor or amplifier T.sub.3 could be a field-effect transistor source-follower.

By way of further example, diode-connected transistors T6 and T6' can be connected to T2 and T2' of FIGS. 3 and 4, respectively, to form current mirror amplifiers for receiving control currents from terminal 14 in like manner to that described above in connection with FIG. 2.