JFET Design Spotlight: Rethinking the Audio Op-Amp

JC - Linear Systems
2 days ago
2 min read

There are a lot of circuits and applications of discrete and integrated component combinations solving a myriad of tasks defined as solutions to a problem. Dad used to tell me that, "The problem isn't the problem; the definition of the problem is the problem." Dr. Gene Slottow at U of I would say, "When you think you're done with the design, throw it out and start over as your unyielding self-critic until you can't anymore, then proceed." Few first circuit iterations are "the one." This was one of those iterations.

Gain control circuits typically use a differential front end and use it as a pivot point as a low-noise front end for an Op-Amp and its inherent massive loop gain, feedback, and convolved transitions of nonlinear elements in between. Compensation is often dicey because of the amount of loop gain being thrown out, as well as the group delay to the feedback node.

Audio doesn't need the PPM level of a typical op-amp having >120 dB of gain. Audio requires linearity and acute transient response without overshoot.

With that in mind, illustrated here is an often-repeated textbook differential input stage with its transconductance gain controlled by emitter current modulation. Preceding an Op-Amp, the low-noise LS312 bipolar pair input voltage must be less than 14 mV for linear operation before the square-law difference becomes nonlinear and it starts to become a comparator.

These collector currents source into a discrete differential transimpedance amplifier with the relatively low loop gain of <60 dB. This particular circuit was originally designed to replace the TA7136 in Boss DS-1 guitar pedals with 9 V battery operation and would work down to about 6.5 V—dead battery mode.

This Op-Amp has a similar asymmetric output impedance character to the TA7136, where slew rate going negative is less than the slew rate going positive, producing slight even-order, octave-related harmonics.

When the amplifier's midpoint voltage is stabilized, it operates linearly open loop with a gain of about 1000, 1 V/mV, and with very low distortion. The LSK389 JFET front end provides high input impedance and linear gain to the single PNP Gm output stage.

This 5-component Op-Amp is ideal for battery audio work with only 1 mA operating current. The VCA operates on 9 V and about 2 mA. Higher input voltages simply require a divider to keep input V <20 mV without emitter degeneration. Differential gain can be modified by a resistor between Q1 and Q2 emitters.

There are no sacred cows in Analog design.

As usual...

— Kirkwood