The gain of an amplifier is controlled by the source-drain impedance of a first field effect transistor having its gate terminal connected to the gate of a second and substantially similar field effect transistor. The second field effect transistor has its source-drain circuit connected in a bridge circuit also including a potentiometer with an adjustable tap providing voltage variations for unbalancing the bridge output. The bridge output operates a differential operational amplifier connected to drive the second field effect transistor for restoring bridge balance. The source-drain impedance of the second field effect transistor, and hence the source-drain impedance of the first field effect transistor, are thereby controlled in proportion to the setting of the aforementioned potentiometer, for adjusting amplifier gain.

A variable FET attenuator uses feedback in a control loop to correct the attenuator automatically for input/output return loss as the attenuator is varied with a control signal input. Both the series and shunt variable FETs are set to the proper conductance to maintain a proper input match with a desired attenuation value. A second, reference attenuator circuit provides feedback control of the primary attenuator circuit through an operational amplifier. The reference circuit is designed to be electrically equivalent to the attenuator circuit in terms of characteristic impedance. The control nodes, and attenuation control signal input and an output from the operational amplifier, are paralleled so that both the reference and primary attenuator circuits behave identically, without having to RF decouple the feedback loop from the attenuator circuit. Preferably, identical bridged-T variable attenuators are used for both the attenuator circuit and the reference circuit. Precise matching is obtained by incorporating both circuits in the same monolithic integrated circuit substrate, together with the operational amplifier.

Linear gain control

Automatic return-loss optimization of a variable fet attenuator

The voltage-to-current converter includes first and second transistors having their respective common electrodes arranged to receive between them the voltage to be converted to current. Their output electrodes have connections to first and second current supply means, as well as direct coupled feedback connections to their respective input electrodes. At least one of these feedback connections is via the follower action of a respective further transistor between its input and common electrodes, and converted output current is available at the output electrode of each further transistor. First and second resistances connect the common electrodes of said first and second transistors to a node supplied quiescent current by a third current supply means, and their combined resistance controls the voltage-to-current conversion gain.

Voltage-to-current converter apparatus

A variable-gain amplifier comprises a transistor with an emitter connected in a degenerative emitter-follower circuit which includes a PIN diode traversed by a current injector in the form of an operational amplifier, the PIN diode forming part of a negative-feedback path extending from the output to the inverting input of the operational amplifier. To achieve a gain characteristic whose logarithm varies linearly with a control voltage, this control voltage is fed to the same inverting input through an amplifier circuit with an exponential characteristic, including another operational amplifier provided with an input transistor. A temperature-compensating circuit, preceding that input transistor, comprises still another operational amplifier having a further transistor inserted in its negative-feedback path. The variable-gain amplifier may be included in one of two conjugate (sum and difference) channels of a monopulse radar.

Variable-gain amplifier

The drain-source conductance of a wide variety of FETs can be made to conform to a predetermined function of a control signal. The control signal is multiplied by a factor and an offset is added or subtracted therefrom to derive a modified control signal. The modified control signal is then applied to the gate of a FET. The factor and the offset are such that the drain-source conductance of the transistor is substantially the predetermined function of the control signal. When a desired attenuator characteristic defined with respect to a FET is to be provided using a FET having characteristics different from those of the reference FET, the control signal to the FET used can be similarly modified so that the desired attenuator characteristics are obtained. A dual path compressor or expander employing an attenuator (which includes a FET) in its further path can be made to conform to a desired compression or expansion characteristic defined with respect to a similar arrangement and with respect to a reference FET conductance control characteristic. This can be accomplished by making the conductance control characteristics of the FET to be used to conform to the desired conductance control characteristic.

Circuits to provide desired conductance characteristics using a FET

Disclosed is an amplifier circuit with gain controlled by a current or a voltage. It is sought to obtain a linear gain as a function of the set value current or voltage, whereas amplifiers with controlled gain usually have a non-linear variation of the gain as a function of gain control voltage. According to the invention, there is proposed an amplifier circuit with the particular feature of having two identical amplifiers with controlled gain: one receives the signal to be amplified and gives an amplified output signal; the other receives a reference voltage or current and gives an output which is compared with a signal with a set value of variation in gain. The result of the comparison is used to automatically control the gain of the second amplifier to adjust its output to a desired value. The first amplifier receives the same automatic control signal as the second one. The second amplifier has its output automatically linked to a set value while, at the same time, having a fixed voltage or current as an input signal. It therefore has a known gain proportionate to the set value. Finally, the first amplifier, which is identical to the second one and receives the same gain control signal, thus has a gain proportionate to the set value, in accordance with what was desired.

Linear gain control

Citations

Automatic return-loss optimization of a variable fet attenuator

Voltage-to-current converter apparatus

Variable-gain amplifier

Circuits to provide desired conductance characteristics using a FET

Amplifier circuit with linearly controlled gain

References

Demodulator circuit with control feedback means

Operational thin film amplifier