Electronic Circuit Design Lab Experiments

To determine the gain and bandwidth of 2-stage RC coupled amplifier.

Sep 24 2023

Aim

Apparatus Required:

Breadboard ,Workstation, Function Generator, Trainer Kit, Patch cords, DC Power supply

Theory:

When two amplifiers are connected in such a way that the output signal of the first serves as the input signal to the second, the amplifier are said to connected in cascade. Amplifier are connected in cascade to extend the gains possible with single stage amplifiers. R.C. Coupling is the most widely used method because it is cheap and provided excellent audio facility over a wide range of frequency. It is usually employed for voltage amplification. Fig. (1) shows two stage R.C. Coupled amplifier. The signal developed across the collector resistor RL of the first stage is coupled to base of second stage through the coupled capacitor C2. This capacitor blocks the DC component of first stage from reaching the base of the second stage. In this way DC biasing of the next stage is not interfered, for this reason capacitor C2 is also called a blocking capacitor.As the coupling from one stage to next is achieved by a coupling capacitor followed by a shunt resistor, therefore the amplifiers are called resistance - capacitance coupled amplifiers resistances R1,R2, RE1 form the biasing and stabilization network. The emitter bypass capactors offers low reactance path to the signal. Without it, the voltage gain of each stage would be lost. It is worth mentioning that the total gain is less than the product of the gains of individual stages. Its reason is that when the second stage follows the first stage, the effective load resistance of the first stage is reduced due to the shunting effect of the input resistance of the second stage. This reduces the gain of the first and second stage will be reduced due to loading effect of the next stage. However, the gain of the third stage which has no loading effect of subsequent stage remains unchanged. The overall gain will be the product of the gains of three stages.

Advantages of R.C Coupling:

It requires no expensive or bulky components and no adjustments. Hence, it is small and inexpensive.
It has excellent frequency response. The gain is constant over the audio frequency range
Its overall amplification is higher than that of the other couplings.
It has minimum possible non-linear distortion because it does not use any coils or transformer which might pick up undesirable signals. Hence, there are no magnetic fields to interfere with the signal.

Procedure:

Connect Audio frequency signal generator across input terminals. Set it at sine wave signal of 100Hz, 10mV - 20mV peak to peak amplitude.
Connect CRO across output of first stage amplifier at red socket above transistor (TR1).
Switch ON the instrument as well as CRO
Throw the feed-back SPDT switch (S2) towards, without feed-back position.
Observe the output signal on CRO. Adjust the output signal through set amplitude potentiometer provided on the front panel. Calculate voltage gain of first stage by using formula:

Voltage Gain ( Av,) = Output Voltage (P-P)/ Input Voltage (P-P).

Repeat the same procedure for with feed-back position. For this throw the feed-back switch (S2) towards with feed-back position. Note down all the observation as shown in Sample Observation Table (1).

Observation Table (1) For 1st Stage Amplifier:

S.no	Frequency	Amplitude with Feedback	Amplitude without Feedback	Gain with Feedback	Gain without Feedback
1	100Hz	0.2V	0.38V	20	38
2	1KHz	0.22V	1.8V	22	180
3	5KHz	0.24V	2.2V	24	220
4	10KHz	0.2V	1.9V	20	190
5	50KHz	0.07V	0.54V	7	54
6	100KHz	0.03V	0.24V	3	24

Plot a graph between Frequency vs Output Gain by taking Frequency along X-axis & Output Gain along Y-axis.

B. Voltage Gain & Frequency response of Second Stage Amplifier:

Connect input signal from Function Generator to blue sockets just below the resistance (R3).
Now connect CRO across output of second stage amplifier.
Throw the feed-back switch (S3) towards, without feed-back position.
Observe the output signal on CRO. Calculate the Output Voltage Gain by using formula

: Voltage Gain ( Av2) = Output Voltage (P-P)/ Input Voltage (P-P)
Repeat the same procedure for with feed-back. For this throw the feed-back switch (S3) towards with feed-back position.

To check the frequency response of amplifier, increase the frequency from 100Hz-100KHz in small steps and everytime note down the output peak to peak amplitude on CRO. Note down all the observation as shown in Sample Observation Table (2).

Observation Table (2) for 2nd Stage Amplifier

S.no	Frequency	Amplitude with Feedback	Amplitude without Feedback	Gain with Feedback	Gain without Feedback
1	100Hz	0.08V	0.16V	8	16
2	1KHz	0.09V	1.2V	9	220
3	5KHz	0.09V	2.2V	9	220
4	10KHz	0.07V	2.2V	7	220
5	50KHz	0.05V	1.0V	5	100
6	100KHz	0.03V	0.5V	3	50

Plot a graph between Frequency vs Output Gain by taking Frequency along X-axis & Output Gain along Y-axis

Voltage Gain & Frequency response of RC Coupled Amplifier

Connect the Red to Red & Blue to Blue dotted sockets through patchcords.
Connect Signal Input to Input sockets and CRO probe to output sockets.
Keep both switches (S2 & S3) towards without feed-back position and take observation as mentioned in previous cases.
Repeat the same procedure for switches (S2 & S3) towards without feed-back position.
Observe the output signal on CRO. Adjust output signal with the help of potentiometer (VR1) provided on the front panel. Note down all the observation as shown in Sample Observation Table (3). Calculate the gain as per formula

Voltage Gain ( Av3) = Output Voltage (P-P)/ Input Voltage (P-P)
Plot a graph between Frequency vs Output Gain by taking Frequency along X-axis & Output Gain along Y-axis.

Viva Questions:

Q.-1. Explain various types of Coupling Amplifier?

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Q.-2. Discuss Negetive Feedback Amplifier?

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Q.-3. Analyze the concept of Frequency response of an Amplifier?

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