audio preamp

Design of an Audio preamplifier using bipolar transistor

This preamplifier is based on the Sziklai bipolar transistor configuration

 Working of Sziklai Transistor Pair

Here we are demonstrating the switching test of the Sziklai pair by giving it a turn-on voltage of 0.75v. Sziklai pair starts conducting at this voltage and LED turns on, that means the turn-on base-emitter voltage for Sziklai pair is equal to normal transistor i.e. 0.75v. 

If we apply pulse input voltage at the base terminal of the NPN transistor Q1 to turn this ON, the PNP transistor Q2 is already in the forward biased state. Therefore, the current travel through the emitter of the transistor Q2 to the collector and emitter of the transistor Q1.As follows:-

 Here are few advantages of Sziklai pair:

-Sziklai pair have lower quiescent current for better linear operations.

-Thermal stability of sziklai pair is superior to the Darlington pair.

-It has faster response time than Darlington pair.

-The turn-on voltage of sziklai pair is equal to a normal transistor, while Darlington takes twice the input voltage.

In order to provide dc base bias for Q1 ;the bias voltage is set by R1 and R4 as follows:-

Sziklai Transistor Pair is arranged with the following components to act as buffer amplifier for no gain.i.e R5 2.2k and R7 2.2k as input and output port as shown.

The video below shows the preamp as unity gain  as shown with the ac voltmeter.

To get a clear idea,let us see on oscilloscope how unity gain looks like.

So as to make a reasonable audio preamplifier, the circuit is redesigned and modified as follows.

 There are two transistors(Sziklai Transistor Pair). They make up a DC feedback pair, with the negative feedback coupled from the collector of Q2 to the emitter of Q1. The input signal is applied via C3 to the base of transistor Q1. The bias voltage for this transistor is set by R1 and R4. 

The output from the first stage is taken from the collector of Q1 and its 22K load resistor R3. Q1’s output is fed to the base of Q2 and the final output signal is taken from its collector via C4. Negative feedback is applied by the 2K2 resistor R5. The 1.5nF capacitor C2 across this resistor ensures stability and reduces interference from radio frequency noise by rolling off frequencies above 48kHz. The overall gain is set by the ratio of R5 and R6. The gain equation is Gain = 1 + (2200/100) = 23 which is approximately 27 dB.

You may change R5 to adjust the overall gain if required. Keep in mind that you may need to adjust C2 as well to maintain the same high frequency roll off. For use with an electric guitar for example, you might try R5 = 10k and C2 = 470pF if there is insufficient gain with the circuit as shown. However you may prefer to leave C2 as shown to reduce noise

The capacitor C3 in series with R6 sets the lower frequency response to 72Hz. This reduces microphone proximity effect and reduces susceptibility to wind and breath noise. If you require a flatter bass response, you may increase the value of C3. A 47 uF capacitor will give a low frequency break point of approximately 33 Hz with 6 dB per octave roll off.

Specification D.C. Input : 6 – 12 V at 2 – 3 mA min.
Maximum output : ~ 2.5 V RMS with 12 V supply
Maximum input : ~ 100 mV RMS (gain = 27 dB)
 Frequency resp. : ~ 70 Hz to 45 kHz –3 dB (circuit as shown )
 THD at 1V 1kHz : < 0.1 %
S/N ratio : > 75 dBA