Common base amplifier and its DC and AC equivalent circuit

Common base amplifier and it’s DC and AC equivalents circuit :

Amplifiers: Amplifiers are the properly biased transistor circuit which are used to amplify input signal (either ac current or voltage) to output signal (ac current or voltage).

The common base amplifier with single transistor in common base mode is shown in figure. In this amplifier the input signal is given to the emitter a transistor through coupling capacitor \(C_1\) whereas the output signal appears across load resistor \(R_1\) with is connected to collector of transistor through another coupling capacitor \(C_2\) . The emitter base junction is forward bias with help of \(V_{EE}\) through emitter resistor \(R_E\) whereas C-B junction resistor \(R_C\) as shown in figure .To analyze the working of CB amplifier we have to take half of dc as well as equivalent circuit.

DC equivalent circuit:

To draw the dc equivalent circuit following steps should be adopted.

• Remove the ac-source i.e. ground ac source to ‘0’.
• Remove all coupling capacitor as they blocks the dc.
• Replace E\B junction by forward bias PN junction diode.
• Replace C\B junction by current source of collector current \(I_C\).

Then, dc equivalent circuit of above CB bias circuit is as shown in below.

In the dc circuit ,

$$ V_{EE}=I_ER_E+V_{BE}$$

$$I_E=\frac{V_{EE}-V_{BE}}{R_E}\dotsm(1)$$

And, also in output section,

$$V_{CC}=I_CR_C+V_{CB}$$

$$V_{CB}=V_{CC}-I_CR_C$$

AC equivalent circuit:

To draw ac-equivalent circuit, Let us adopt following steps as:

• All the dc series are shorted as they are treated as ground.
• All the coupling capacitor \(C_1\) and \(C_2\) mode closed as they allow as signal.
• The emitter base junction is replaced by ac emitter resistance of junction diode \(r_{ac}=r_e’\) where \(r_e’=\frac{25mv}{I_E}\) and \(I_E\) is dc emitter current.
• The C\B junction is replaced by current source of\( i_C\). As seen from input as source , it is shared by \(R_E\) and \(r_e’\) and then seen from output side, collector current \(i_C\) is shorted by \(R_C\) and \(R_L\) i.e. \(\frac{R_E}{r_e’}\) and \(R_C \shortparallel R_L\)

Principle characteristics operations:

• Input resistance (\(r_{in}\)):

The resistance offered by input section is input resistance (\(r_{in}\) and also called as \(r_{in}\),stage

$$r_{in}=R_E\shortparallel r_e’$$

$$r_{in}=\frac{R_E.r_e’}{R_E+re’}$$Since,\(R_E>r_e’\)

$$r_{in}=\frac{R_E.r_e’}{R_E}=r_e’$$Whch is very low.

• Ac load resistance (\(r_L\)):

It is the resistance offered by output section of the circuit as shown in figure,\(r_L=R_C\shortparallel R_L\)

$$r_L=\frac{R_C.R_L}{R_C+R_L}$$ Which is large.

• Current gain:

Current gain is given by the ratio of output current to the input current i.e.

\(A_i\)=\(\frac {otput current}{input current}\)=\(\frac{i_C}{i_e}\)

Since, \(i_C=\alpha i_e\)

$$A_i=\frac{\alpha i_e}{i_e}$$

$$\therefore A_i=\alpha$$This shows that there is no current gain in common base amplifier.

• Voltage gain(\(A_v\)):

The voltage gain of C-B amplifier is defined as ratio of output voltage to the input voltage i.e.

\(A_v\)=\(\frac{output votage}{input voltage}\)

$$A_v=\frac{i_C.r_e}{i_e.r_{in}}$$

$$A_v=\alpha =\biggl(\frac{r_l}{r_e’}\biggr)$$ Since, \(\alpha\)=\(\frac{r_l}{r_e’}\)

This shows that voltage gain of C-B amplifier is large.

Power gain:

The power is given by produced of current gain and voltage gain. This means it is ratio of output power to input power.

\(A_p\)=\(\frac{output power}{input power}\)

=(current gain)\(\times\)(voltage gain)

=\(A_i\times A_v\)

=\(\alpha \times \frac{r_l}{r_e’}\)

This shows that the power gain is large in CE amplifier. CB is used as voltage as well as power gain.

Circuit operation:

When the input signal is not given then no ac output signal appears across the load resistor. During ‘+’ ve half cycle of input ac signal:-

• The input emitter current decreases due to opposite dc emitter current(\(I_E\).
• This decreases the biasing potential (\(V_{BE}\)).
• This causes decreases in base current (\(i_B\)).
• Then collector current is also decreases in drop across \(R_C\) i.e.\(i_cR_C\) decreases.
• Finally, the output voltage \(V_{CB}\) decreases on \(V_{CB}=V_{CC}-i_CR_C\) i.e. in increase, decrease $$ v_{in}\uparrow i_e \downarrow v_{BE}\downarrow i_B\downarrow i_C\downarrow R_C \downarrow v_{CB}\uparrow$$
• This shows that during ‘+’ve going half cycle of input signal ‘+’ve going output signal is appears. This means there is no phase inversion occurs in C-B amplifier i.e. input signal and output signal are in same phase.

Characteristics of CB-amplifier:

Following are features of CB-amplifier :

• Input resistance very low(\(25\Omega-50\Omega\).
• The output resistance is large(\(\sim\) upto 1500 k\(\Omega\)).
• There is no current gain i.e. current gain slightly less thsn unity.
• The voltage gain is large.
• The power is also large.

Hence, due to power as well as voltage gain the CB amplifier is used for gain of power and voltage.

Uses:

• The amplifier is used for impedance matching i.e. low input impedance can match with high output resistance.
• The amplifier is used as voltage amplifier and power amplifier.
• This amplifier is used as pre- amplifier to amplify the weak signal as it’s input resistance is very low.

References:

(1)Theraja, B.L. Basic Electronics. N.p.: S.Chand, n.d. Print.

(2)C.L.Arora. Refresher Course in Physics. Vol. II and III. N.p.: S.Chand, 2006. Print.

(3)Malvino. Electronic Principles. N.p.: Tata McGraw-Hill, n.d. Print.

(4)N.Nelkon and P.Parker. Advanced Level Physics. 5th ed. N.p.: Arnold Heinemann, n.d. Print.

(5)Priti Bhakta Adhikari,Diya Nidhi Chaatkuli, Ishowr Prasad Koirala. A Textbook of Physics (2nd Year). N.p.: Sukunda Pustak Bhawan, 2070. Print.