Comparison of CE, CB and CC Configurations

Comparison of CE, CB and CC Configurations: It is useful to study the variation of A_I, A_V and R_i with change in load resistance R_L for each of the three configurations.

Similarly, we may study variation of R₀ with change of source resistance R_S. Figure 1, 2 and 3 plots for each of the three configurations, current gain A_I, voltage gain A_V and input resistance R_i respectively as function of load resistance R_L for the typically transistor whose h-parameters are given in Table 4.1.

From these curves we get the following information:

(A) CE Amplifier | Comparison of CE, CB and CC Configurations

A_I is high for $R_L < 10 K\Omega$

A_V is high for usual values of R_L. Typical value of R_L is $1 k\Omega$

R_i is medium (about $10^3\Omega$ ) and varies nominally with variation of R_L.

R₀ is moderately high and varies appreciably with variation of R_S.

Uses: CE amplifier is capable of providing moderately high values of both A_I and A_V. Hence it is popularly used.

Table 1 Approximate Conversion Formulas for h-parameter
$h_{ic} = h_{ie}$	$h_{ib} = \dfrac{h_{ie}}{1+h_{fe}}$
$h_{fc} = (1+ h_{fe})$	$h_{fb} = -\dfrac{h_fe}{1+h_{fe}}$
$h_{rc} = 1$	$h_{rb} = \dfrac{h_{ie}\times h_{oe}}{1+h_{fe}}-h_{re}$
$h_{oc} = h_{oe}$	$h_{ob} = \dfrac{h_{oe}}{1+h_{fe}}$

(B) CB Amplifier | Comparison of CE, CB and CC Configurations

A_I < 1 and its magnitude decreases with increase of R_L beyond $10^4\Omega$ , reaching a value of about 0.2 for $R_L = 10^7\Omega$ .

A_V is high for usual value of R_L.

R_i is low ( $<10^3\Omega$ ), being lowest of all the three configurations.

R₀ is high $(>10^6\Omega for R_L> 4 k\Omega)$ , being highest of all the three configurations.

Uses: CB amplifier is not used as an amplifier for providing voltage or current gain. It is used only:

For matching a very low impedance source to the high input impedance of the main amplifier.
As a noninverting amplifier with A_V > 1.
For driving a high impedance load.
As a constant current source for use in a sweep (time-base) voltage generator or to charge a capacitor linearly with time.

(C) CC Amplifier | Comparison of CE, CB and CC Configurations

A_I is high, almost equal to that of CE amplifier for $R_L < 10 k\Omega$

A_V <1

R_i is high, being highest of all the three configurations.

R₀ is low, being lowest of all the three configurations.

Uses: It is used as a buffer amplifier between a high impedance source and a low impedance load.

Table 2 gives the value of A_I, A_V, A_P, R_i, and R₀ for a typical transistor (h-parameters given in table 1) for all the three configurations using $R_S = 1000\Omega$ and $R_L = 4 k\Omega$ . Actually, these values of A_I, R_i, A_V, R₀ and A_P are picked up randomly.

Table 2: Performance Schedule of Typical Transistor in CE, CB and CC Configurations
Quantity	CE	CB	CC
A_I	-45	0.98	46
A_V	-172	169	0.99
A_P	7817	166	46
A_i	$1055 \Omega$	$23 \Omega$	$187 K\Omega$
R₀	$52 \Omega$	$1.28 M\Omega$	$41 \Omega$