Why Use Discrete Transistor Circuitry?
Douglas Self

   By Sloane   Categories: Audio Equipment

This excerpt deals with small-signal design using discrete transistors, specifically BJTs. Many things found in standard textbooks are skated over quickly. It concentrates on audio issues, and gives information that I do not think appears anywhere else, including the distortion behavior of various configurations.

Why Use Discrete Transistor Circuitry?

Circuitry made with discrete transistors is not obsolete. It is appropriate when:

1.)     a load must be driven to higher voltages than the op-amp can sustain between the supply rails. Op-amps are mostly restricted to supply voltages of [1]18 or [1]20 V. Hybridconstruction amplifiers, typically packaged in TO-3 cans, will operate from rails as high as [1]100 V, but they are very expensive, and not optimized for audio use in parameters like crossover distortion. Discrete op-amps provide a viable alternative;

2.)    a load requires more drive current, because of its low impedance, than an op-amp can provide without overheating or current limiting, e.g. any audio power amplifier;

3.)    the best possible noise performance is required. Discrete bipolar transistors can outperform op-amps, particularly with low source resistances, say 500 U or less. The commonest examples are moving-coil head amps and microphone preamplifiers. These almost invariably use a discrete input device or devices, with the open-loop gain (for linearity) and load-driving capability provided by an op-amp which may itself have fairly humble noise specs;

4.)    the best possible distortion performance is demanded. Most op-amps have Class-B or -AB output stages, and many of them (though certainly not all) show clear crossover artefacts on the distortion residual. A discrete op-amp can dissipate more power than an IC, and so can have a Class-A output stage, sidestepping the crossover problem completely;

5.)    it would be necessary to provide a low-voltage supply to run just one or two op-amps. The cost of extra transformer windings, rectifiers, reservoirs, and regulators will buy a lot of discrete transistors. For example, if you need a buffer stage to drive a power amplifier from a low impedance, it may be more economical, and save space and weight, to use a discrete emitter-follower running from the same rails as the power amplifier. In these days of autoinsertion; fitting the extra parts on the PCB will cost very little;

6.)    purely for marketing purposes, as you think you can mine a vein of customers that don’t trust op-amps.

When studying the higher reaches of discrete design, the most fruitful source of information is paradoxically papers on analog IC design. This applies with particular force to design with BJTs. The circuitry used in ICs can rarely be directly adapted for use with discrete semiconductors, because some features such as multiple collector transistors and differing emitter areas simply do not exist in the discrete transistor world; it is the basic principles of circuit operation that can be useful. A good example is a paper by Erdi, dealing with a unity-gain buffer with a slew rate of 300 V/ms [1]. Another highly informative discourse is by Barry Hilton [2], which also deals with a unity-gain buffer.

Excerpt from Douglas Self’s Small Signal Audio Design.

About the Book:

Small Signal Audio Design is a unique guide to the design of high-quality circuitry for preamplifiers, mixing consoles, and a host of other signal-processing devices. Learn to use inexpensive and readily available parts to obtain state-of-the-art performance in all the vital parameters of noise, distortion, crosstalk and so on. Focusing mainly on preamplifiers and mixers this practical handbook gives you an extensive repertoire of circuits that can be put together to make almost any type of audio system.

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