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#### CHAPTER 14 SEMICONDUCTORS

Transistor as a device

1. Transistor as an amplifier

Amplification of dc voltage: In the active region transistor behaves like an amplifier. For Vo versus Vi curve, the slope of the linear part of the curve represents the rate of change of the output with the input. It is negative because the output is VCC – ICRC and not ICRC. Hence, the output voltage of the CE amplifier decreases as input voltage increases. In this case the output is said to be out of phase with the input. If we consider ΔVo and ΔVi as small changes in the output and input voltages then $\frac{\mathrm{\Delta }{\mathrm{V}}_{\mathrm{o}}}{\mathrm{\Delta }{\mathrm{V}}_{\mathrm{i}}}$ is called the small signal voltage gain AV of the amplifier.

If the VBB voltage has a fixed value corresponding to the midpoint of the active region, the circuit will behave as a CE amplifier. The dc base current IB would be constant and corresponding collector current IC will also be constant.

The voltage gain AV can be expressed in terms of the resistors in the circuit and the current gain of the transistor as follows.

We have, Vo = VCC – ICRC

Therefore, ΔVo = 0 – RC Δ IC

Similarly, from Vi = IBRB + VBE

ΔVi = RB ΔIB + ΔVBE

But ΔVBE is negligibly small in comparison to ΔIBRB in this circuit.

Also, the dc voltage VCE = VCC - ICRC would remain constant. The operating values of VCE and IB determine the operating point, of the amplifier.

Amplification of ac signal

If a small sinusoidal voltage with amplitude vs is superposed on the dc base bias by connecting the source of that signal in series with the VBB supply, then the base current will have sinusoidal variations superimposed on the value of IB. As a consequence the collector current also will have sinusoidal variations superimposed on the value of IC, producing in turn corresponding change in the value of VO. We can measure the ac variations across the input and output terminals by blocking the dc voltages by large capacitors. Let us superimpose an ac input signal vi, to be amplified, on the bias VBB (dc). The output is taken between the collector and the ground.

To start with let us assume that vi = 0. Then applying Kirchhoff’s law to the output loop, we get,

Vcc = VCE + ICRC

And from the input loop, we get,

VBB = VBE + IB RB

When vi is not zero, we get,

VBE + vi = VBE + IB RB + ΔIB (RB + ri)

Or vi = ΔIB (RB + ri) = r ΔIB

The power gain Ap can be expressed as the product of the current gain and voltage gain. Mathematically

Ap = βac × AV

• The transistor is not a power generating device. The energy for the higher ac power at the output is supplied by the battery.