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
