Advertisements
Advertisements
Question
Derive an expression for voltage gain of the amplifier and hence show that the output voltage is in opposite phase with the input voltage.
Solution
For a transistor to act as an amplifier, operating point is fixed in the middle of its active region. An ac input signal vi is superimposed on bias VBB (dc). The output is taken between collector and ground. Applying Kirchhoff’ law to the output loop.
VCC=VCE+ICRC
VBB=VBE+IBRB
Vi≠0
Then, VBB+Vi=VBE+IBRB+ΔIB(RB+ri)
`because r_i=((DeltaV_(BE))/(DeltaI_B))_(V_(CE))`
`therefore v_i=DeltaI_B(R_B+r_i)`
`=rDeltaI_B`
`beta_(ac)=(DeltaI_C)/(DeltaI_B)=i_c/i_b`
It is current gain denoted by Ai.
Change IC due to change in IB causes a change in VCE and the voltage drop across resistor RC, because VCC is fixed.
∴ ΔVCC = ΔVCE + RCΔIC = 0
⇒ ΔVCE = − RCΔIC
Change in VCE is the output voltage Vo.
∴ Vo = ΔVCE = − βac RCΔIB
Voltage gain of amplifier
`A_V=V_o/V_i=(DeltaV_(CE))/(rDeltaI_B)`
`=-beta_(ac)R_c/r`
Negative sign represents that the output voltage is in opposite phase to input voltage.
APPEARS IN
RELATED QUESTIONS
Write the functions of three segments of a transistor.
Draw a circuit diagram of a p-n-p transistor and explain how it works as a transistor amplifier
Draw a circuit diagram of a transistor amplifier in CE configuration.
Define the terms : Input resistance How are these determined using typical input and output characteristics?
Draw a circuit diagram for studying the input and output characteristics of a n-p-n transistor in common emitter configuration. Using the circuit, explain how input, output characteristics are obtained
Draw a labelled diagram of a common emitter amplifier, showing waveforms of the signal voltage and output voltage.
Gautam went for a vacation to the village where his grandmother lived. His grandmother took him to watch 'nautanki' one evening. They noticed a blackbox connected to the mike lying nearby. Gautam's grandmother did not know what that box was. When she asked this question to Gautam, he explained to her that it was an amplifier.
(i) Which values were displayed by the grandmother? How can inculcation of these values in students be promoted?
(ii) What is the function of an amplifier?
(iii) Which basic electronic device is used in the amplifier?
Figure shows a block diagram of a detector for amplitude modulated signal. Identify the boxes ‘X’ and ‘Y’ and write their functions.
In common emitter amplifier, the current gain is 62. The collector resistance and input resistance are 5 kΩ and 500 Ω respectively. If the input voltage is 0.01 V, the output voltage is:
A pnp transistor is used in common-emitter mode in an amplifier circuit. A change of 40 µA in the base current brings a change of 2 mA in collector current and 0.04 V in base emitter voltage. Which of the following is the correct value of the input resistor Rin?
