The Working Theory of an RC Coupled Amplifier in Electronics

Amplification is a process of increasing the signal strength by increasing the amplitude of a given signal without changing its characteristics.  An RC coupled amplifier is a part of a multistage amplifier wherein different stages of amplifiers are connected using a combination of a resistor and a capacitor. An amplifier circuit is one of the basic circuits in electronics.

An amplifier that is completely based on the transistor is basically known as a transistor amplifier. The input signal may be a current signal, voltage signal, or a power signal. An amplifier will amplify the signal without changing its characteristics and the output will be a modified version of the input signal. Applications of amplifiers are of a wide range. They are mainly used in audio and video instruments, communications, controllers, etc.

Single Stage Common Emitter Amplifier:

The circuit diagram of a single-stage common emitter transistor amplifier is shown below:

Circuit Explanation

A single-stage common emitter RC coupled amplifier is a simple and elementary amplifier circuit.  The main purpose of this circuit is pre-amplification that is to make weak signals to be stronger enough for further amplification. If designed properly, this RC coupled amplifier can provide excellent signal characteristics.

The capacitor Cin at the input acts as a filter which is used to block the DC voltage and allow only AC voltage to the transistor. If any external DC voltage reaches the base of the transistor, it will alter the biasing conditions and affects the performance of the amplifier.

 R1 and R2 resistors are used for providing proper biasing to the bipolar transistor. R1 and R2 form a biasing network that provides necessary base voltage to drive the transistor inactive region.

The region between the cut off and saturation region is known as the active region. The region where the bipolar transistor operation is completely switched off is known as a cut-off region and the region where the transistor is completely switched on is known as saturation region.

Resistors Rc and Re are used to drop the voltage of Vcc. Resistor Rc is a collector resistor and Re is emitter resistor. Both are selected in such a way that both should drop Vcc voltage by 50% in the above circuit. The emitter capacitor Ce and emitter resistor Remakes negative feedback for making the circuit operation more stable.

Two-Stage Common Emitter Amplifier:

The circuit below represents the two-stage common emitter mode transistor amplifier where resistor R is used as a load and the capacitor C is used as a coupling element between the two stages of the amplifier circuit.

Circuit Explanation:

When input AC. the signal is applied to the base of the transistor of the 1^st stage of RC coupled amplifier, from the function generator, it is then amplified across the output of the 1st stage. This amplified voltage is applied to the base of the next stage of the amplifier, through the coupling capacitor Cout where it is further amplified and reappears across the output of the second stage.

Thus the successive stages amplify the signal and the overall gain is raised to the desired level. Much higher gain can be obtained by connecting a number of amplifier stages in succession.

Resistance-capacitance (RC) coupling in amplifiers are most widely used to connect the output of first stage to the input (base) of the second stage and so on. This type of coupling is most popular because it is cheap and provides a constant amplification over a wide range of frequencies.

Transistor as Amplifiers

While knowing about different circuits for RC coupled amplifiers, it is important to know about transistors basics as amplifiers. The three configurations of the bipolar transistors that are commonly used are common base transistor (CB), common emitter transistor (CE), and common collector transistors (CE).  Other than transistors, operational amplifiers can also be used for amplification purposes.

• Common emitter configuration is commonly used in the audio amplifier application because common-emitter has a gain that is positive and also greater than unity. In this configuration, the emitter is connected to ground and has high input impedance. Output impedance will be medium. Most of these types of transistor amplifier applications are commonly used in RF communication and optical fiber communications (OFC).

• The common base configuration has a gain less than unity. In this configuration, the collector is connected to the ground. We have low output impedance and high input impedance in the common base configuration.

• Common collector configuration is also known as emitter follower because the input applied to the common emitter appears across the output of the common collector. In this configuration, the collector is connected to the ground. It has low output impedance and high input impedance. It has a gain almost equal to unity.

Basic Parameters of a Transistor Amplifier

We need to consider the following specifications before choosing the amplifier. A good amplifier must have all the following specifications:

• It should have a high input impedance
• It should have high stability
• It must have high linearity
• It should have high gain and bandwidth
• It must have high efficiency

Bandwidth:

The range of frequency that an amplifier circuit can amplify properly is known as the bandwidth of that particular amplifier. The curve below represents the frequency response of the single-stage RC coupled amplifier.

The curve which represents the variation of gain of an amplifier with frequency is called the frequency response curve. The bandwidth is measured between the lower half power and upper half power points. P1 point is lower half power and P2 is upper half power respectively. A good audio amplifier must have a bandwidth from 20 Hz to 20 kHz because that is the frequency range that is audible.

Gain:

The gain of an amplifier is defined as the ratio of output power to the input power. Gain can be expressed either in decibel (dB) or in numbers. The gain represents how much an amplifier is able to amplify a signal given to it.

The below equation represents a gain in number:

G= Pout/Pin

Where Pout is the output power of an amplifier

The pin is the input power of an amplifier

The equation below represents a gain in decibel (DB):

Gain in DB= 10log (Pout/Pin)

Gain can also be expressed in voltage and current. The gain in voltage is the ratio of the output voltage to the input voltage and gain in current is ratio of output current to the input current. The equation for gain in voltage and current is shown below

Gain in voltage= output voltage/ input voltage

Gain in current= output current/ input current

High Input Impedance:

Input impedance is the impedance that is offered by an amplifier circuit when it is connected to the voltage source. The transistor amplifier must have high input impedance in order to prevent it from loading the input voltage source. So that is the reason for having high impedance in the amplifier.

Noise:

Noise refers to unwanted fluctuation or frequencies present in a signal. It may be due to the interaction between two or more signals present in a system, component failures, design flaws, external interference, or maybe by virtue of certain components used in the amplifier circuit.

Linearity:

An amplifier is said to be linear if there is any linear relationship between the input power and the output power. Linearity represents the flatness of the gain. Practically it not possible to get 100% linearity as the amplifiers use active devices like BJTs, JFETs, or MOSFETs, which tend to lose gain at high frequencies due to internal parasitic capacitance. In addition to this, the input DC decoupling capacitors set a lower cutoff frequency.

Efficiency:

The Efficiency of an amplifier represents how an amplifier can utilize the power supply efficiently. And also measures how much power from the power supply is gainfully converted at the output.

Efficiency is usually expressed in percentage and the equation for efficiency is given as (Pout/ Ps) x 100. Where Pout is the power output and Ps is the power drawn from the power supply.

A Class A transistor amplifier has 25% efficiency and provides excellent signal reproduction but the efficiency is very low. Class C amplifier has efficiency up to 90%, but the signal reproduction is bad. Class AB stands in between class A and class C amplifiers so it is commonly used in audio amplifier applications. This amplifier has an efficiency of up to 55%.

Slew Rate: 

The slew rate of an amplifier is the maximum rate of change of output per unit time. It represents how rapidly the output of an amplifier can be changed in response to change in the input.

Stability:

Stability is the capacity of an amplifier to resist oscillations. Usually, stability problems occur during high-frequency operations, close to 20 kHz in case of audio amplifiers. The oscillations may be of high or low amplitude.

I hope this basic yet important topic of electronic projects has been covered with ample information. Here is a simple question for you- For what purpose is a common collector configuration used and why?

Give your answers in the comment section below.