What is a multistage amplifier?

The multistage amplifier is generally a voltage or power amplifier. The multistage amplifier has a number of single stage amplifier that are cascaded together. Since a single stage amplifier output is not useful, they are being replaced with multistage amplifiers or multistage transistor amplifiers. The multistage amplifier are used where the gain required is high or the input signal is very small such that only a single stage amplifier cannot amplify it.

Figure1 shows the schematic arrangement of a two-stage amplifier. Here there are two stages connecting the coupling device. The output voltage of the first stage is connected to the coupling device and the output of the coupling device is given to the second stage, i.e., the input of the second stage.

The coupling devices can be a capacitor or a transformer. The connection of two amplifier via a coupling devices is called as a cascading and a multistage amplifier.

The basic functions of a coupling device are as follows:

1. To move AC voltage from one stage output to the next stage input.
2. To block DC voltage and pass AC voltage from one stage to another.

Types of multistage amplifier

1. Resistance-Capacitance (RC) Coupling Multistage Amplifier
2. Transformer Coupling Multistage Amplifier
3. Direct Coupling Multistage Amplifier

Resistance-Capacitance (RC) Coupling Multistage Amplifier

Construction

Figure 2 shows the constructional diagram of Resistance - Capacitance (RC) Amplifier. Here in amplifier circuit, there are two transistor connected with Common-Emitter configuration. The common supply V_cc is connected. The resistance R₁ and R₂ are used as potential divider, resistance R_e is used to stabilize and biasing the network. The emitter by-pass capacitor C_e is providing low reactance path to the signal.

Here, C_c is the coupling capacitor. It serves as a link between two amplifiers or stages. It is used to create DC interference between two stages. R_L stands for load impedance. C_in is connected to the input capacitor at the first step. It connects the AC signal to the base of the transistor.

Working

When an AC signal is applied to the first amplifier stage of transistor's base, it is amplified and appears at the collector load R_L, which is then transmitted to the next amplifier stage via the coupling capacitor C_C. This is sent into the next stage, amplified output appears over the collector load once more. As a result, the signal is increased stage by stage.

The total gain is less than the sum of each amplifier stage gains. When a second amplifier stage is added to follow the first, the shunting effect of the second stage's input resistance lowers the first stage's effective load resistance. As a result, only the gain of a multistage amplifier's second stage remains constant.

Frequency Response

The above figure shows the graphical representation of frequency response of RC Coupling Amplifier. It shows the relationship between Frequency and Voltage gain. Here the Voltage gain is constant between frequencies 50 Hz to 20 kHz. The voltage gain decreases for the frequencies below the 50 Hz and above 20 kHz.

Transformer Coupling Multistage Amplifier

Construction

The figure shows schematic diagram of Transformer Coupling Amplifier. The two transistor are in common connection with each other called common emitter configuration. The common supply V_cc is connected. The resistance R₁ and R₂ are potential divider, resistance R_e is stabilizing and biasing the network. The emitter by pass capacitor C_e is providing low reactance path to the signal.

Here, Transformer T₁ is used as a coupling device between two stages. The collector load is replaced by the primary winding of transformer T₁, and Secondary winding is connected to potential divider and base of second transistor i.e it is input for second amplifier stage.

Working

When an alternating current signal is applied to the input of the first amplifier stage of transistor's base, it is amplified by the transistor and appears at the collector to which the transformer's primary is connected.

Here, transformer T₁ is the coupling device and it is changing the impedance that means load resistance of the stage can be reflected to high load resistance past stage. Therefore, the primary voltage is transferred according to the turns ratio of secondary side. The excellent impedance matching between two stages is provided by the coupling transformer T₁.

Frequency Response

The figure shows schematic graphical representation of Frequency response of Transformer Coupling Amplifier. Here, the graph consist of the X-axis represents the Frequency and Y-axis represents the Voltage gain.

From the graph it can be said that, the voltage gain of amplifier is constant for small range of frequency. At low frequency the voltage gain is decreases, and also high frequency gain is reduces.

Direct Coupled Multistage Amplifier

In direct coupled amplifier there is no coupling devices, the transistors are directly connected to each other so it is called as Direct Coupled Amplifier. It is also known as three stage Amplifier.

Construction

The figure shows schematic diagram of three transistors i.e T₁, T₂, and T₃ respectively. In amplifier circuit, the output of transistor 1 is connected to the input of transistor 2. In first stage the transistor will be NPN transistor. In second stage the transistor act will be PNP transistor and so on. It happens because the current is increased in collector and variations are cancelled one transistor by decreasing other one.

Working

When transistor 1 is amplified then output appears across collector resistor R_c of transistor 1. This output is given to the base of transistor 2, then it is amplified. Then, it gives a signal to next one, this completes all the stages of direct coupled amplifier.

Frequency Response

The figure shows the graphical representation of frequency response of Direct Coupled Amplifier. The graph consist of X-axis and Y-axis. The X-axis represents Frequency and Y-axis shows the Voltage gain. Here, the voltage gain remain constant at certain stage frequency, after that frequency goes increasing and voltage gain decreases.

Related Concepts

Impedance Coupling

In Impedance coupling the coupling devices are inductance and capacitance are used. So it is called as Impedance Coupling.

The impedance coupling method uses a coil, whose impedance is determined by the inductance and frequency of the signal. This approach is not well-known, yet it is commonly employed.

Context and Applications

1. They have great audio fidelity across a wide frequency range.
2. Voltage amplifiers are widely utilized.
3. Generally used for impedance matching.
4. Used for power amplification.
5. When maximum amplification power transfer is required.
6. Low frequency amplification.
7. Low current amplifications.

In each of the expert exams for undergraduate and graduate publications, this topic is huge and is mainly used for

• Bachelor of Technology in Electrical and Electronics Department
• Bachelor of Science in Physics
• Master of Science in Physics

Common Mistakes

1. Make sure that input voltage is correct for the amplifier.
2. Connection must be done properly.
3. Check the rating of equipment before providing the supply.
4. Make sure that coupling device is connected properly.
5. Make sure that ground is given at the proper places in the circuit.
6. Do not get confused with the use of different coupling capacitors.

Practice Problems

Q1. What are types of Multistage amplifier?

A. Resistance-Capacitance (RC) Coupling

B. Direct Coupling

C. Transformer Coupling

D. All of the above

Answer: D

Explanation: Depending upon the type of coupling there are resistance capacitance coupling amplifiers, direct coupling amplifier and transformer coupling amplifier.

Q2. In RC coupling amplifier the coupling device is...........

A. Capacitor

B. Inductor

C. Resistor

D. Transistor

Answer: A

Explanation: Capacitor is a coupling device for RC coupling amplifier. It has wide frequency range.

Q3. In transformer coupling ..................... is coupling device.

A. Capacitor

B. Transformer

C. Resistor

D. Transistor

Answer: B

Explanation: As name of coupling the transformer is used for transformer coupling.

Q4. In Direct Coupling, the coupling is provided between two amplifier by.............

A. Capacitor

B. Transformer

C. No Coupling (Direct Coupling)

D. Transistor

Answer: C

Explanation: In direct coupling there is no coupling device, transistors are directly connected to each other.

Q5. In Impedance coupling the coupling devices are ................ and .................

A. Capacitance and Inductance

B. Transformer and Resistor

C. Transistor and Capacitor

D. None

Answer: A

Explanation: In Impedance coupling the coupling devices are inductance and capacitance are used. So it is called as Impedance Coupling.