Electronic Design

Fast, Stable Wideband FET Amplifier

The design idea presented here is a gain-trimmable wideband FET amplifier with very good speed, high input impedance, and excellent dc stability under all conditions.

Transistors Q1 and Q2 constitute a simple, high-speed FET input buffer. Q1 functions as a source-follower, with the Q2 current-source load setting the drain-source channel current. Q3 and Q4 form a simple discrete gain stage, while Q5 and Q6 form the output stage.

The OP-77 op amp contributes dc feedback to stabilize the circuit. This is accomplished by comparing the filtered circuit output to a similarly filtered version of the input signal. The amplified difference between these signals then is used to set the bias of Q2, and hence Q1’s channel current. This forces Q1’s VGS to whatever voltage is required to match the circuit’s input and output potentials. The RC network in the OP-77’s output prevents it from seeing any high-speed edges coupled through Q2’s collector-base junction.

The voltage gain of this amplifier is equal to the RA/RB ratio. The RB adjustment allows the gain to be precisely set to the desired value—usually between 1 and 10 (RB = 1k and RB = 100 Ω, respectively):

VOUT = VIN * RA / RB

With the components shown in the figure (i.e., gain of 1):

  • Output slew rate > 1000 V/µs
  • 0-dB bandwidth = 20 MHz
  • −3-dB bandwidth = 60 MHz

At a gain of 10, the 0-dB point occurs at 12 MHz, and the −3-dB point occurs at 40 MHz.

This circuit provides more than adequate performance for driving video cable, high-speed data converters, etc.

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