Op Amp And Two JFETs Form A Voltage-Controlled Amplifier

April 28, 2005

A simple linear voltage-controlled amplifier can be constructed with one op amp and two JFETs (see the figure). The amplifier can achieve an 80-dB dynamic control range with less than ±0.2% linearity error for 0 V ¾ VC ¾ 0.8

Henry Santana

This article is part of the Ideas for Design in our Series Library

A simple linear voltage-controlled amplifier can be constructed with one op amp and two JFETs (see the figure). The amplifier can achieve an 80-dB dynamic control range with less than ±0.2% linearity error for 0 V ≤ V_C ≤ 0.8 × V_P.

1. The JFETs in this voltage-controller amplifier act as linear resistors because the op amp constrains their drain-to-source voltages to a few tenths of a volt.

R is chosen to be much larger than the R_DS(ON) of the JFETs. This permits a large range of input voltages to be applied without driving the JFETs into a nonlinear region. If, for example, R_DS(ON) = 100 Ω and R is chosen to be 10 kΩ , a 10-V input signal (V_IN) will result in a V_DS of less than 100 mV.

The value of k can be chosen to supply gain. Although p-channel JFETs are shown, n-channel JFETs will work as well for negative control voltages (V_C). Also, it should be noted that the drain and source terminals are interchangeable in this application.

The JFETs should be chosen for low R_DS(ON) and high I_DSS. If a forward gate voltage is possible, a current limiter in the gate is advisable.

For the largest dynamic range, the resistor connected from V_IN to the noninverting input can be adjusted for a minimum output with V_C = 0 V. The trim range is about ±5% of R. For the lowest temperature sensitivity, maintain good thermal coupling between Q1 and Q2.

If V_C = V_IN, the circuit will perform as a voltage squaring circuit where V_O = —(k/V_P) × V_IN² for unipolar V_IN (quadrant 4 for p-JFET and quadrant 2 for n-JFET).