AMPLIFIERS 

SINGLE STAGE BJT AMPLIFIERS 

1.1. Gain and Impedance

The primary function of an amplifier is to provide voltage gain, current gain or power gain, 

depending on the application in which the amplifier is used. The gain of a single stage BJT 

amplifier depends not only on the gain parameters ∝ and β of the transistor itself, but also on 

resistances or impedances external to the BJT and internal resistances of the BJT. 

It is important to remember that the term “impedance” applies to a complex quantity that has 

both magnitude and phase angle. 

1.2. Voltage Gain 

It is appropriate, at this point, to define the various gains of an amplifier. The voltage gain, of 

an amplifier is defined as the ratio of the AC Voltage Vo developed across the load resistance R 

to the AC Input Voltage Vi

 applied to the amplifier input terminals. Hence, by definition 

A v = (Vo / Vi) ---------- (1–1) 

It is quite possible that Vo and Vi will not be in phase, in which case the voltage gain will be a 

complex quantity. 

1.3 Current Gain 

The current gain Ai of an amplifier is defined as the operation of the AC current I o delivered to 

the load, to the AC current Ii

 supplied to the amplifier input. Hence, by definition 

Ai

 = (I o/Ii) ------------- (1–2) 

Here again, if Io and Ii are not in phase, the current gain will be a complex quantity. 

1.5. Power Gain 

The power gain Ap of an amplifier is defined as the ratio of the AC output power delivered to 

the load, to the AC input power supplied to the amplifier by the source. Hence, by definition 

Ap = (Po/Pi) ------------ (1–3) 

can also be written as Ap = Av x Ai

 --------- (1–4) 

indicating that the power gain is the product of the voltage gain and the current gain. 

Input Resistance

Another important parameter in amplifier gain calculations is the input resistance of the

amplifier, defined as the ratio of AC input voltage Vi to AC input current, Ii

. Hence, by

definition

Ri

 = (Vi / Ii) ----------- (1–5)

If the input voltage and current are not in phase, their ratio will be a complex number and we

then speak of input impedance.

The gain of a single–stage BJT amplifier depends upon the input resistance (or

impedance) of the amplifier and the load resistance connected to its output terminals.

Consider Fig. 1.1 which shows an AC amplifier with two input terminals and two output

terminals. This amplifier can be single BJT in the CE configuration, or it can be a number of

BJTs connected in series (in cascade). The input resistance of this amplifier is represented by

Ri

. Signal source Vs having source resistance RS develops input voltage Vi

 across the

amplifier input terminals and supplies input current Ii. By ohm’s law we write

Vi

 = IiRi --------------- (1–6)

Representing the amplifier as a current generator I o which develops an output voltage Vo

across load resistor RL, we can write

Vo = I oRL ----------------- (1–7)

Dividing Eq.(1–7) by Eq.(1–6), we obtain

(Vo / Vi) = I oRL / IiRi -------------- (1–8)

we recognize the ratio Vo / Vi as the voltage gain AV, and the ratio I o Ii

 as the current gain Ai,

so Eq. (1–8) reduces to

AV = AiRL / Ri

 -------------- (1–9)

1.8 Eq. (1–9) is a fundamental expression for the voltage gain of any amplifier, It

states that the voltage gain of an amplifier is equal to the product of the current gain and the

ratio of load resistance to input resistance.