Class A preamplifier

Class A preamplifier: Nowadays, the design of preamplifiers is almost invariably based on integrated circuits. However, there are still many constructors de who scribed like a discrete design here is for them.

The design of the amplifier is symmetrical. the differential input stages dual transistors Tland T2 are used. The differences in current amplification between these make bias correction at the input stage essential and this is provided by T12. The required reference voltage source is provided by D2. Preset P2 sets the output voltage to 0 V. The only drawback of the correction stage is that an input coupling capacitor, C1, becomes essential. However, with the value of this capacitor as shown, the cut-off frequency is only 1.5 Hz, so in practice, this is no real problem.

The input stages drive a push-pull amplifier that consists of T5 and T6. To increase the stability of this amplifier, its gain is limited by R18-R19.

The output stage is a Conventional emitter follower, T7, and T10. A current source, con-the Quiescent T8, ensures constancy of For best through the emitter follower. “1 sides of Tr best performance, the plane 7 and T9, and e of T8 and T10 should be joined together after some heat conducting paste has been applied to them. A brass clamp to make the mechanical coupling secure is advisable, but not absolutely essential.

The quiescent current through T9 and Teo is set to 20 mA with P. The value of the current may be measured on the basis’ of the voltage drop across R22 and R23. Capacitor C2 serves to improve the response of the stage and to suppress over-shoot. The effective bandwidth is thus reduced to 2.4 MHz, which is more than ample for audio applications.

For optimum performance, all transistors operate in Class A, which results in the relatively large quiescent current of 40 mA. To ensure correct feedback, the earths of input and output, R1, R2, R18, and R19, as well as the negative supply line must be taken to one earthing point only. Some measuring results (with Ub= ±15 V. 1 V r.m.s. output and a 1. fl load):

Gain: 16 dB

(input sensitivity 150 mV)

Slew rate 200 V us-1

Signal-to-noise ratio: 100 dB (unweighted)

Third harmonic distortion <0.00025% (20 Hz-20kHz).

Class A preamplifier Schematic diagram

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