Class 12 Physics Semiconductor Electronics Transistor as an amplifier

Transistor as an amplifier

Amplifier – It is a device which is used for increasing the amplitude of the alternating voltage, current or power

Amplifier using n-p-n transistor in common emitter configuration is shown below:

Circuit Diagram: • The emitter is common to both the input and output circuit
• The input circuit is forward biased with the battery VBB and the output circuit is reverse biased with the battery Vcc
• RL is the load resistor connected in the collector circuit

Input and output:

Working:

Case 1 – No AC signal voltage is applied to the input circuit

• When no AC signal voltage is applied to the input circuit, let the emitter current, base current and collector current be IE, IB and Ic
• By applying Kirchhoff’s law, we know, IE = IB + Ic
• From the output circuit, we find, VCC = VCE + Icily Hence, VCE = VCC - IcRL

Case 2 – When AC signal voltage is applied to the input circuit

• There will be a change in the emitter-base voltage and hence the emitter current
• As emitter current changes, collector current changes
• In equation, VCE = VCC - IcRL as the collector current changes, the collector voltage VCE changes accordingly as VCC is fixed
• The change in the collector voltage appears to be amplified output of the input variation

Current and Voltage gain

• The change in the output current by the input current is called current gain indicated by βac

βac = Change in Ic  / Change in IB = ic / ib

• The change in the output voltage by the input voltage is called voltage gain indicated by Av

Av = vo / vi = δVCE / r δIB  where r is the input resistance

= - βac RL / r

The negative sign indicates that the output voltage is opposite with the phase of input voltage

Power gain

• The power again is the product of the current gain and voltage gain.
• Ap = βac * Av
• The power gain is also defined as the ratio of change in output power to the change in input power

Problem

For a CE-transistor amplifier, the audio signal voltage across the collector resistance of 2 KΩ is 2 V. Suppose the current amplification factor of the transistor is 100, find the input signal voltage and base current, if the base resistance is 1KΩ.

Solution

Ro = 2 KΩ = 2000 Ω ; Vo = 2V ; βac = 100 ; Ri = 1KΩ = 1000 Ω

(a) Av = Vo / Vi = βac R0 / Ri

which is 2 / Vi = 100 * (2000 / 1000) ; Hence, Vi = 0.01 V

(b) Ib = Vi  / Ri

= 0.01 / 1000 = 10 * 10-6 A

Problem

Two amplifiers are connected one after the other in series (cascaded).  The first amplifier has a voltage gain of 10 and the second has a voltage gain of 20. If the input signal is 0.01 V, calculate the output of ac signal’

Solution

Total voltage gain is AV = AV1 * AV2  = δVo / δVi

δVo = δVi * AV1 * AV2

= 0.01 * 10 * 20 = 2V

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