AMPLIFIER GAIN CONTROL CIRCUIT INCLUDING DIODE BRIDGE

• US 3,582,807 A
• Filed: 07/28/1969
• Issued: 06/01/1971
• Est. Priority Date: 07/28/1969
• Status: Expired due to Term

• Current Assignee: John L. Addis
• Original Assignee: John L. Addis
• Inventors: Addis, John L.
• Application Number:
  US04/845,471
• Time in Patent Office: 673 Days
• US Class Current: 330/145
• CPC Class Codes: H03G 1/0052 using diodes

Abstract

A gain control circuit for a DC coupled wide-band amplifier is described in which a diode bridge is employed to adjust the gain by providing a variable resistance between bridge outputs connected to such amplifier whose value changes with bias current flowing through such bridge. Bias current for the diode bridge is varied by changing the bias voltages applied to two bias resistors at inputs on opposite sides of such bridge, equally in opposite directions with respect to the quiescent DC voltage of the bridge outputs. This is achieved by an operational amplifier having an input connected to one side and its output connected to the other side of the bridge and a reference voltage on its other input equal to the DC voltage on the bridge outputs to maintain the bias current in the two bias resistors equal. Two of the bridge diodes are of much larger junction area than the other two diodes to improve the linearity of the bridge resistance. A pushpull amplifier using the gain control diode bridge had a frequency response of DC to 630 megahertz and a rise time of 0.55 nanoseconds.

Claims

1. A direct coupled amplifier circuit having a variable gain, the improvement comprising:

gain control means including a diode bridge connected at its output to the amplifier for varying the gain of said amplifier in response to changes in the resistance of said bridge;

said bridge including a pair of first diodes having their cathodes connected in common at one input of the bridge and a pair of second diodes having their anodes connected in common at another input of said bridge, and with the anodes of one of said first diodes connected in common with the cathode of one of said second diodes at each of the outputs of said bridge;

bias means connected to the inputs of said bridge for supplying a bias current through said diodes so that they are quiescently biased conducting in a substantially linear region of their operating characteristic, and for changing the bias voltage applied to said inputs equally in opposite directions and thereby varying the bias current to change the resistance of said bridge between its outputs while maintaining the quiescent DC output voltage level of said bridge constant;

said bias means including a pair of bias resistors each having one terminal connected to a different one of the inputs of said bridge, and variable bias voltage means for applying different bias voltages to the other terminals of said bias resistors and for varying said bias voltages in such a manner as to maintain the voltage drop between one bridge input and the bridge outputs equal to the voltage drop between the other bridge input and said bridge outputs in order to cause the same bias current to flow through each bias resistor and to prevent any of said bias current from flowing through the outputs of the bridge; and

said variable bias voltage means including an operational amplifier means formed by a phase inverter amplifier of high internal gain having a first input connected through a coupling resistor to the common terminal of one bias resistor and the output of a variable DC voltage source, said inverter amplifier having its output connected to the other bias resistor, and a negative feedback resistor connected from the output to the input of the inverter amplifier.

2. An amplifier circuit in accordance with claim 1 which includes reference means for applying a DC reference voltage to a second input of the operational amplifier of the polarity as its output, said reference voltage being approximately equal to the quiescent DC supply voltage on the outputs of said bridge.

3. An amplifier circuit in accordance with claim 2 in which the variable DC voltage source includes a potentiometer connected at its end terminals to different DC voltages having its movable contact connected as the output of said voltage source and having one end terminal connected to a source of DC voltage equal to said reference voltage.

4. An amplifier circuit in accordance with claim 3 which also has a temperature compensation means including the parallel impedance of a voltage divider resistor, connected between the other end terminal of the potentiometer and the other source of DC vOltage.

5. An amplifier circuit in accordance with claim 4 which also has a bridge balance means including another potentiometer connected at its end terminals across the series circuit of the bias resistors and the bridge with the movable contact of said other potentiometer connected to one of the outputs of said ridge.

6. A direct coupled amplifier circuit having a variable gain, the improvement comprising:

gain control means including a diode bridge connected at its output to the amplifier for varying the gain of said amplifier in response to changes in the resistance of said bridge;

said bridge including a pair of first diodes having their cathodes connected in common at one input of the bridge and a pair of second diodes having their anodes connected in common at another input of said bridge, and with the anode of one of said first diodes connected in common with the cathode of one of said second diodes at each of the outputs of said bridge;

said first and second pairs of diodes having one diode with a much larger rectifying junction than the other diode of the pair, and the two diodes of similar large area junctions are connected in common to one of the bridge outputs; and

bias means connected to the inputs of said bridge for supplying a bias current through said diodes so that they are quiescently biased conducting in a substantially linear region of their operating characteristic, and for changing the bias voltage applied to said inputs equally in opposite directions and thereby varying the bias current to change the resistance of said bridge between its outputs while maintaining the quiescent DC output voltage level of said bridge constant.

7. An amplifier circuit in accordance with claim 6 in which the diodes are metal to semiconductor rectifying junction diodes which have little charge storage.

8. A direct coupled amplifier circuit having a variable gain, the improvement comprising:

an input amplifier stage having a pair of push-pull outputs;

an output amplifier stage having a pair of push-pull inputs;

a coupling means coupled said input and output stages, including a first line connected from one push-pull output to one push-pull input and a second line connected from the other push-pull output to the other push-pull input;

gain control means including a bridge of unilateral conducting elements connected at its outputs between said first and second lines for varying the gain of said amplifier circuit in response to changes in the resistance of said bridge;

said bridge including four unilateral conducting elements each having first and second terminals with a pair of first elements having their first terminals connected in common at one input of the bridge and a pair of second elements having their second terminals connected in common at another input of said bridge, and with the second terminal of one of said first elements connected in common with the first terminal of one of said second elements at each of the outputs of said bridge; and

bias means connected to the inputs of said bridge for supplying a bias current through said elements so that they are quiescently biased conducting in a substantially linear region of their operating characteristic, and for changing the bias voltage applied to said inputs equally in opposite directions and thereby varying the bias current to change the resistance of said bridge between its outputs while maintaining the quiescent DC output voltage level of said bridge constant.

9. An amplifier circuit in accordance with claim 8 in which the coupling means is a transmission line of substantially uniform characteristic impedance formed by said first and second lines.