Power amplifier electronic circuits-tube audio rf amplifiers

Electronic Amplifier Circuits-Power amplifier, audio amplifier designs and circuits

November 17th, 2016

Class A RC coupled amplifier

Class A RC coupled amplifier theory and design.

This article is about a Class A RC coupled amplifier, its theory of operation, working and design. RC coupled Class A amplifier is one of the most simple and commonly used small signal amplifier. RC coupled means the input signal is coupled to the input of the amplifier through an RC network. Capacitor connected at the input forms the “C” and the collector resistance forms the “R”. To be more precise, in a multi stage amplifier design, the capacitor connecting the output of the first stage to the input of the second stage will be the “C” and the collector resistor of the transistor in the second stage will be the “R”.

Class A operation means the amplifying element ie; the transistor conducts for the full cycle of the input signal. That means the complete cycle of the input waveform will be available at the output. No part of the input information is lost in the output. Circuit diagram of a typical Class A RC coupled amplifier is given below.

RC coupled amplifier circuit diagram.

RC coupled amplifier circuit

Function of each component in the above circuit diagram is as follows. Cin is the input decoupling capacitor. It blocks any DC component that may be present in the input signal from entering the amplifier. If DC components in the input are not blocked, it may change the biasing conditions of the amplifier and this will alter the performance.

R1 and R2 are biasing resistors. Their function is to give proper forward bias to the base of the transistor. The forward base-emitter voltage of a Silicon transistor is 0.7V. That means the transistor starts conducting only after the base-emitter voltage has risen above 0.7V. If biasing is not given, all parts of the input waveform where amplitudes are below 0.7V will be lost in the output. The function of the biasing resistors are to keep the base of the transistor at a suitable voltage so that even the least amplitude parts of the input wave form are conducted.

RC is the collector resistor. Its primary job is to set the collector current of the transistor. Usually 40% of the Vcc drops across the RC.

RE is the emitter resistance. Usually 10% of Vcc is dropped across RE. RE in conjunction with RC makes sure that the operating point is located at the center of the load line.

CE is the emitter by-pass capacitor. Its job is to by-pass the ac component of the emitter current. if this capacitor is not there, the ac component of the collector current will get added to the base-emitter voltage (VBE) of the transistor and this will disrupt the biasing conditions.

Cout is the output capacitor. Its job is to de-couple the dc component of the collector current from the output. If this is not done, the dc component of the collector current will flow freely through the low impedance load and this is a waste. Also the dc component may affect the succeeding stage if any.

Q1 is the transistor itself. It is the active element that performs the task of amplification. The transistor performs this job by amplifying the small fluctuation in the collector current by a factor called amplification factor into a large fluctuation in the collector current.

RL is the load resistor. It is used to set the gain of the RC coupled amplifier.

Designing the RC coupled amplifier.

Next part shows the design of an RC coupled amplifier with following parameters. Few parameters of the amplifier has to be set previously set before designing. They are Gain(Av), collector current(IC), supply voltage(Vcc), and input frequency(F)

Here we have IC=5mA, Vcc=12V , Av=10 and F=1KHz. β of the transistor is assumed to be 100. Now lets see the design of each components.

1.RC and RE.

RC and RE must be so selected that 40% of Vcc drops across RC and 10% of Vcc drops across RE. remaining 50% drops across the transistor itself.

That means RC=0.4(Vcc/IC) and RE=0.1(VCC/IC)

Substituting the values of Vcc and IC we get

RC= 960Ω ≅ 1K 

RE= 240Ω ≅ 220Ω

2.R1 and R2

Current through R1 must be 10 times the base current. Also voltage across R2 must be equal to the sum of the VBE of the transistor and the voltage drop across emitter resistor RE. So we have,


IR1=IR2=10 (IC/β)=10(5mA/100)=50uA


VBE=0.7V and VRE=ICxRE=5mAx220Ω=1.2V







R1=VR1/IR1 and R2=VR2/IR2. Substituting the values in hand we get,

R1=202.2K ≅200K



3.Cin, CE and Cout.

Cin should be so selected that its impedance should be 1/10th of the input resistance of the amplifier.

XCin = (1/10) Rin


re is the resistance looking in to the emitter terminal of the transistor and here it is 25mV/IE

also IC≅IE

so re=25mV/5mA = 5Ω

substituting the values in the equation for Rin we get


so Rin=492Ω

XCin =1 / 2π F Cin

From this we get Cin=1/2πFXcin   and F is 1KHz

Substituting the values we got in to the above equation we get

Cin= 3.2uF≅ 3.3uF


CE should be so selected  that its impedance is equal to 1/10th of the emitter resistor (RE).

XCE=1 / 10 (RE)

XCE=1 / 2π F CE

CE=1 / 2π F XCE

substituting the values we got into the above equation we get

CE=7.2uF≅ 10uF



Cout should be so selected that its impedance  is equal to 1/10th of the output resistance of the amplifier.

XCout=1/10th Rout   ; where Rout is the output resistance of the amplifier.

and we know that Rout= RC   the collector resistance.

XCout= 1 / 2π F Cout

therefore Cout= 1 / 2π F XCout

Substituting the values we got in to the above equation, we get the value of Cout as

Cout= 16.5uF ≅ 22uF

4. Load resistor RL.

Load resistor RL can be used to set the gain of the transistor. We know that the voltage gain of a common emitter transistor amplifier, Av= -(rc / re)

Av = – (rc / re)

rc = RC // RL

re = 25mV / IE

Whe know the voltage gain Av required is 10 and re is 5Ω.

Using these values in the above equations we get

RL= 52.74Ω ≅ 56Ω











January 12th, 2011

Hi-fi Amplifier Circuit

Circuit Description

Figure gives the circuit diagram of 2X12 W Hi-Fi Amplifier. This amplifier circuit is based on TDA 2616 IC which is the core part of this circuit. The TDA2616 is a stereo power amplifier IC. It is a 9-lead single-in-line (SIL9) plastic power package (SOT131). This amplifier IC finds applications in mains fed amplifier circuits such as stereo radio, tape and television. It has gain balance for both of the channels and hi-fi according to standards such as IEC 268 and DIN 45500. This IC has special inbuilt circuit for the suppression of noise signals at the inputs, during switch-on and switch-off. This avoids click sounds during power on and power off.

Simple and less number of components make this circuit suitable for portable power amplifier circuits. A well regulated and filtered +/- 12 V dual power supply is used in this circuit to provide at least 2 A continuous current. The circuit can deliver a maximum output of 12 W power on 8 Ohm speaker for each channel at +/- 12 V dual supplies. This Hi-fi amplifier circuit can complement your audio system with ceiling speakers for home theater.

Circuit Diagram

Hi-Fi Amplifier Circuit

Hi-Fi Amplifier Circuit


  • Use a good quality PCB to assemble this particular circuit.
  • IC TDA 2616 is from Philips.
  • All capacitors except C10 & C9 are ceramic type.
June 11th, 2010

555 Timer Police Siren Circuit

Circuit Description

Police siren circuit given here is a circuit based on NE 555 timer IC. This circuit uses two NE 555 ICs (IC1 and IC2) and which are the core working parts of this circuit. Here two timer ICs are wired to work as astable multivibrators. But these two timer ICs are working at different frequencies. IC1 is slow astable multivibrator operating at a frequency of 20Hz and a duty cycle of 50% whereas IC2 works at 600Hz as fast astable multivibrator. The output of IC1 is fed to the control pin (pin5) of IC2. By this connection, the output frequency of IC2 will be modulated with the output frequency of IC1.

The circuit can be powered from a DC supply voltage ranging between 6V and 15V. Power supply used here is 12V DC supply. The loudness of output can be further increased by inserting a power amplifier circuit at the output stage of this circuit.

Circuit Diagram

555 Timer Police Siren Circuit

555 Timer Police Siren Circuit


  • Use a Perf board to assemble this circuit.
  • The output frequency of IC2 is controlled by the output of IC1.
  • One NE 556 IC can be used instead of two NE 555 ICs.

Use speaker with rating 64 ohm, 500mW.