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A slightly different kind of voltage amplifier

Posted 12th December 2016 at 10:44 AM by rjm
Updated 21st December 2016 at 04:50 AM by rjm

(which is a current feedback amplifier)

Lately I have been looking again for a discrete transistor voltage gain amplifier for line level duty that isn't just another op amp.

I keep coming back to variations on this circuit. It's a diamond buffer input with current mirrors in the mid-section strapped around a voltage divider / feedback loop that provides the gain. This backs out into either another diamond buffer or, in the revised version below, a standard Sziklai output stage which can be more easily scaled up as needed for a headphone amp for example.

An offset bias adjust circuit could be added to trim the output offset voltage. Or use a coupling cap. It's a few hundred mV otherwise.

Circuit gain is R4/R6, approximately. The total value of R4+R6 should be kept about 20 kohms. C1 is a compensation capacitor. Circuit gain as shown is 14 dB, -3 dB at 250 kHz.

As with all CFA, the choice of the feedback resistance R4, R6 is critical. The gain can be varied from about 3 dB (R4 = 10k, R6 = 10k) to the 14 dB shown. Setting the gain higher, either by reducing R6 or increasing R4 runs into problems with stability and bandwidth, respectively.

The simulated distortion figure is -95 dB for 0 dB output into 600 ohms. The PSRR is about 35 dB over the audio bandwidth (at output, for A=14 dB).
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