Controlling Amplifier Gain with Voltage: An Innovative Circuit Design

Controlling Amplifier Gain with Voltage: An Innovative Circuit Design

Engineering Professor Unveils Novel Op Amp and JFET Circuit for Gain Control

In the realm of audio engineering, controlling the gain of an amplifier has traditionally been achieved using variable resistors, such as volume controls. However, an engineering professor has recently introduced a groundbreaking circuit design that allows for the control of amplifier gain with voltage. By utilizing a pair of operational amplifiers (op amps) and a pair of matched Junction Field-Effect Transistors (JFETs), this innovative circuit offers a new approach to gain control. In this article, we will explore the intricacies of this circuit and its potential implications for the field of audio engineering.

The Circuit Analysis

The analysis of this circuit is relatively straightforward, as long as we assume that the op amps are not in saturation. In this case, we can safely assume that the op amp will adjust its output to equalize the input terminals. The left-hand op amp in the circuit has one input grounded, causing the output to drive the first JFET in order to maintain a 0V potential at the negative terminal. By examining the circuit, we can observe that the current through resistor R1 must be equal to the current through the first JFET, which is determined by the control voltage (which is negative) divided by R1.

Matching JFETs for Optimal Performance

A crucial aspect of this circuit design lies in the matching of the JFETs. The gate voltage on the first JFET’s gate is identical to that of the second JFET. Since the JFETs are matched, we can assume that the current through the second transistor will be the same as the first transistor. This synchronization allows for precise control of the gain. It is important to ensure that the control voltage remains within the linear region of the JFETs to maintain optimal performance. In essence, the JFETs function similarly to variable resistors in this circuit.

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Video Demonstration and Simulation

While the concept may initially appear confusing, a video demonstration can provide clarity and enhance understanding. By visually observing the circuit in action, viewers can gain a deeper comprehension of its functionality. Additionally, for those seeking a more in-depth understanding, simulating the circuit can be a valuable tool. Various simulation software options are available, allowing engineers to explore and analyze the circuit’s behavior in a controlled environment.


The of a circuit design that enables the control of amplifier gain with voltage marks a significant advancement in the field of audio engineering. By utilizing op amps and matched JFETs, engineers can now achieve precise gain control in a novel and efficient manner. This innovative circuit opens up possibilities for further exploration and experimentation in amplifier design. As technology continues to evolve, it is through groundbreaking designs like this that the boundaries of what is possible in audio engineering are continually pushed.