Experiment 6: Design a RC Coupled CE Transistor Amplifier

Objective:-

                  1. Connect the circuit diagram, Measure the DC Bias Conditions (V_CE, V_BE,I_C).
                  2. Measure the AC Gain, Signal Handling Capacity and the Bandwidth 

Equipments Required:- 

1) multimeter  2) Bread Board 3) 12V dc power supply 4)Function Generator      5) DSO

Components Required :-

1)Resistors(R1=68K,R2=18k,Rc=1.2K,Re=1K)) 2)Capacitors C_in=C_out=1uF, C_e=33uF

 3) Transistor no- BC548 (NPN)

Theory:-

An amplifier is a device that is used to amplify input voltage or current.  The RC coupled CE transistor amplifier is a popular scheme of cascading two or more amplifiers to achieve the required levels of amplification. The inter-stage coupling is done by using a RC network, hence the name.The variation of gain with frequency of an amplifier is called the frequency response characteristics of the amplifier.

The bandwidth of the RC amplifier is the difference between upper cut-off frequency and lower cut-off frequency. It represents the range of frequencies that the amplifier is most effective in amplifying. The cut-off frequency points are determined from the frequency response graph, where the gain (dB) is 3 dB lower than the maximum gain.

For a  transistor to act as an amplifier, it must be properly biased. i.e. its emitter base junction must be forward biased & collector base junction must be reverse biased. The transistor is operating in the active region,  is verified by measuring the dc bias conditions.

Circuit diagram for measuring dc bias conditions:-

Connect the components as shown in the circuit diagram. Measure the DC bias conditions.

Circuit Diagram:-

Vce = (Approx 25% to 75% of  Vcc) ,  Vbe= (Approx. .6V  → .7V), Ic=( Approx.  VRc /Rc)

Circuit diagram to find the frequency response curve :-

Procedure:- Connect the components as shown in the diagram

1.   Apply a 50mv peak-to-peak signal from the function generator to the input of the amplifier and measure output voltage in the CRO. Note down the Vpk-pk readings for different frequencies keeping the input signal constant.

2.   Calculate the voltage gain in dB for each frequency and plot the frequency response curve in a semi-log graph paper. From it find the bandwidth.

3.   Signal Handling Capacity:- In the mid frequency, select any frequency, go on increasing the input, observing the output in the oscilloscope. At some value of input, the output waveform starts showing distortion. That input value  in volts or mv  is called the signal handling capacity.

Table

Vin	Frequency(f)	Vout	Gain (Vout/Vin)	Gain in dB
50mv pp	50Hz
	100Hz
	500Hz
	1Khz
	5khz
	10KHz
	20Khz
	50KHz
	100KHz
	200KHz
	500KHz
	700KHz
	1MHz

Expected Graph-   Plot the graph in a semilog graph paper