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Few Transistor amplifier circuits

admin

March - 4 - 2011

11
Comments

This article is a list of various types of
amplifier circuits built using Transistors alone.This includes a Class AB
amplifier using 4 transistors, then we have a headphone amplifier circuit and
finally a low power amplifier using transistor. This article is solely dedicated
to publish more transistor amplifier circuits. So you may keep visiting this
post in future for more updates.

List of  Transistor
Amplifiers

1. Class
AB 4 Transistor Amplifier

2.
Headphone Amplifier using Transistors

3. A
Low cost amplifier circuit  using Transistor

These are circuits that can be used in a lot of
low scale applications. The main feature of these circuits is that they all
are just basic designs and the components used can be easily obtained from
your junk box. More over the value of the components are not very critical and
slight variations in it will not affect the performance.

1. 4 Transistor Class AB
amplifier.

Description.

A very simple and easy to build class AB audio
amplifier using four transistors is shown here. In class AB operation each
output device conducts more than half of the input signal cycle. Up to 78%
efficiency is possible with class AB designs and cross over distortion is
reduced. The circuit shown here is suitable for small radio receivers, audio
players, intercom, telephone etc.

Transistor Q1 with its associated components is
wired as a pre amplifier stage. The audio input is coupled to the base of Q1
through resistor R1 and capacitor C1. Resistor R3 provides collector to base
bias for Q1 and C3 is an AC by pass capacitor for the collector resistor R4.
Collector to base biasing is a good method of biasing for circuits like this as
it provides enough negative feedback, prevents thermal runway and stabilizes the
operating point. The second stage is the driver stage for the push pull pair. Q2
with its associated components perform this job. This stage is also collector to
base biased and its input is coupled to the output of the preamplifier stage
using capacitor C2. Resistor R8 limits the collector current of Q2.The third
stage is the class AB push pull section comprising of transistors Q3 and Q4.
Diodes D1 and D2 provides the bias voltage for the push pull stage. The output
of the amplifier is coupled to the loud speaker through the capacitor C4. C5 and
C6 are power supply filter capacitors.

Circuit diagram of 4 transistor
amplifier.

4 Transistor Class AB amplifier

Notes.

The circuit can be assembled on a vero board.

K1 can be an 8 ohm/5W speaker.

C6 must be grounded near to the Q1 and C5 must be grounded near to the
loud speaker ground. This reduces noise.

Use 5V DC for powering the circuit.

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To Top of List

2. Class A Headphone
amplifier.

Description.

This is the circuit diagram of a Headphone
amplifier operating in the class A push pull mode. In class A mode the output
device (transistors) conduct over the entire input signal cycle. The maximum
possible efficiency for Class A operation is 50% and it further reduces when
capacitive coupling is used. But the advantages of Class AB amplifier are no
cross over distortion, high fidelity and low harmonic distortion. These
amplifiers are most suitable for low power applications.

In the circuit transistor Q1 works as the
preamplifier. Resistors R6 and R7 provides potential divider biasing for Q1.
Audio input is coupled to the base od Q1 through capacitor C2, resistor R9 and
POT R10. Emitter of Q1 is coupled to the base of Q2 through resistor R3. Diodes
D1 and D2 provides bias voltage for Q2. Base of Q3 is directly coupled to the
collector of Q1. Resistor R5 limits the collector current of Q2 and Q3. C4 and
C5 are power supply filter capacitors. Output of the amplifier is coupled to the
head phone using the capacitor C3.

Circuit diagram of head phone
amplifier.

Head phone amplifier Class A

Notes.

The circuit can be assembled on a vero/perf board.

Power supply can be anything between 6 to 24V DC.

I used 12V DC for powering the circuit.

Z1 can be a 100 ohms or higher head phone.

Electrolytic capacitors voltage rating must be according to the supply
voltage you use.

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3.Low power transistor
amplifier circuit

Description.

A very simple low power three stage transistor
amplifier circuit is shown below. First and second stages are pre amplifiers and
third is the output stage. Collector to base biasing is employed for each stages
and it is enough for circuits like this.

Collector to base
biasing: Consider the first stage.R1 is the collector resistor
and R2 is the base resistor. This type of biasing provides some negative
feedback, avoids thermal runaway and stabilizes the operating point. When the
temperature increases the collector current also increases. This increases the
voltage drop across the collector resistance R1 and this decreases the voltage
drop across base resistor R2. As a result, base current drops which in turn
drops the collector current and thermal runaway is avoided. Stabilization of
operating point is achieved by the prevention of variation in collector current
with temperature.

When a transistor amplifier is operating,
the collector voltage will be phase opposite to the input signal. Since base
resistor R2 is connected between collector and base, some fraction of the phase
opposite collector voltage is fed back to the input (base) through the base
resistor R2 and thus negative feedback is achieved. Negative feed back reduces
the voltage gain of the amplifier but it improves the stability.

In the circuit collector of Q1 is coupled to the
base of Q2 using the capacitor C2. R3 is the biasing resistor for Q2 and R4 is
its collector resistor.Collector of Q2 is coupled to the base of Q3 using the
capacitor C3. R5 is the biasing resistor for Q3 and speaker is connected as the
collector load. Capacitors C4 and C5 are filters. C1 is the input DC decoupling
capacitor.

Circuit diagram of low power transistor
amplifier.

Low power transistor amplifier

Notes.

The circuit can be assembled on vero or perf board.

Use 9V DC for powering the circuit. A 9V PP3 battery is fine.

All electrolytic capacitors must be rated 10 or 15V.

An optional protection diode can be added in series to the positive supply
line. This protects the circuit from accidental polarity reversals.

Type number of the transistors are not very critical. Replacement will not
affect the performance to a great extend.

Powering the circuit using a mains adapter will induce some noise.

Speaker impedance can be 64 ohms or more.

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To Top of List

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11 Responses to “Few Transistor amplifier circuits”

masoud says:

November 3, 2013 at 3:56 am

thank you .do i 2n3055 used?

Reply

Parsa says:

January 8, 2013 at 12:51 pm

Hi everyone
I constructed the circuit of the 4 transistor class AB
amplifier and it worked well. I just have a small question;
Why is C3 used
to bypass R4? I mean why do we need to bypass it? and why to bypass it with
such a small capacitor?
Thank u in advance
Parsa

Reply

mike says:

December 17, 2012 at 4:08 am

the class AB circuit doesn’t work, te currents are correct but the load
isn’t. I don’t understand why people will publish this when it ain’t
working.

Reply

Mihai says:

July 4, 2012 at 11:17 am

Hi , I just constructed Circuit diagram of low power transistor
amplifier
the last one from this page…I don,t know why , my speaker pulsing
without any signal…when I apply signal from my guitar pickup to see if the amp
working, the sound coming out very noisy…after the vibration of the string
going down a little, the amplifying effect stops and the rhythmical pulsing
going further ( like a heart)…

Why this happening? Please give me an advice where to look for problem in
this schematic?

Regards
Mihai

Reply

Geoff says:

March 4, 2012 at 3:50 pm

Hi all, I have a hearing impairment which makes listening to tv frustrating
for my wife and I as I need it louder than normal, but, when the volume alters
from soft speech to loud music etc we are constantly battling with the remote,
is there such thing as an auto volume adjuster amplifier to keep the tv sound
on a constant level to bring relief to my wife and I.
Many thanks in
anticipation.
Geoff

Reply

Nico says:

January 30, 2012 at 4:13 am

where should I connect c3? across r3 and at the diode? or across r3 to
r7?

Reply

Jonathan says:

March 8, 2011 at 5:20 am

In the first ckt, C3 should be across R7 and about 100pF, C2 can be 1 mF,
and R9,R10 should be 1/2 to 1 ohm. K1 will only see about 1/4 watt at 5V
supply.

Reply

Jonathan says:

March 8, 2011 at 5:10 am

Peltier cooler. Some surplus dealers have them, but they ain’t cheap. You
might find one junked somewhere. If the cool ing unit doesn’t work, take it
apart; sometimes if one of the little (1/8th inch long) elements is corroded;
you can jump across it with a wire.

Reply

KI SENEVIRATHNA
says:

March 7, 2011 at 10:19 am

An eny one have a idea about light witch is giving by sun?O K,I will give a
idea but i dont know how it get,enyway derect sun light can it get u r room
?Yes i think using 2 pc of magnificent glass or one reflector tyoe glass and
sutable fiber optics cable.Yes may be im pool but sumone who knos about
transfering light withen fiber optic cable and to be manufact the device to
joint magnificent glass and fiber optic cable.
Do u think it will work? why
not think about how satalite antenna working.Out side audio vedio signal come
to TV withen electric cable.Cannot wefollow this theory for this matter.I hope
some day some one will do this type of lighting system,Im sure.what u
think?Plese write u r comments.Im inviting to every body who intrusted about
this. Thank u circuits today.

Reply

KI SENEVIRATHNA
says:

March 7, 2011 at 9:59 am

Hi,Wish u a good day every one.I need to know what is the construction of
portable 12v dc powered cooler.And what is the name of tha device which is
giving cool & heat,when we give the power.

Reply

Mark says:

March 5, 2011 at 2:24 am

In the first circuit the capacitor should be placed across R3 not R4.

Reply

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