1. DR. S. & S. S. GANDHY COLLEGE OF ENGINEERING AND TECHNOLOGY
SUB:-ELECTRONIC DEVICES AND CIRCUITS
Group
ENROLLMENT NO.
NAME
ADITI AGRAWAL
VISHAL AKBARI
AJAY AMBALIYA
Guided by.
Parul mem.

2. Topic Base biased amplifier Introduction
Base biased amplifier circuit explanation with example

3. COUPLING CAPACITOR
Figure shows an AC voltage source connected to a capacitor and a resistor. Since the impedance of the capacitor is inversely proportional to frequency, the capacitor effectively blocks dc voltage and transmits ac voltage. When the ac frequency is high enough, the capacitive reactance is much smaller than the resistance. In this case, almost all the ac source voltage appears across the resistor. When used in this way, the capacitor is called a coupling capacitor because it couples or transmits the ac signal to the resistor.

4. fig

5. Good coupling : Xc < 0.1R
Cont.
Coupling capacitors are important because they allow us to couple an ac signal into an amplifier without disturbing its Q point.
For a coupling capacitor to work properly, its reactance must be much smaller than the resistance at the lowest frequency of the ac source.
Good coupling : Xc < 0.1R
Hence reactance should be at least 10 times smaller than the resistance at the lowest frequency of operation.
When this condition is satisfied

6. Cont.. __________ __________ Z = √ R2 +Xc2 = √ R2 + 0.01 R2 = 1.005R
__________ __________
Z = √ R2 +Xc2 = √ R R2 = 1.005R
Hence we can approximate all coupling capacitors as an ac short.
Since dc voltage has a frequency of zero, the reactance of coupling capacitor is infinite at zero frequency. Hence we can approximate the capacitor as open.

7. Q-Why Biasing is required?
These values are the DC Bias conditions for the circuit. Biasing is required so that the transistor can react to small input voltages which are not large enough to turn on the transistor (recall that the transistor starts to conduct at approximately 0.5 V - the voltage at the base emitter - and the normal base emitter operating voltage is about 0.7 V).

8. INTRODUCTION -Base Bias Circuit
Operation
Initially with no base current flowing the output or collector voltage VOUT = VC = VCC the supply voltage. The resistor RB is connected from VCC to the base of the transistor. Base current will flow in the transistor causing the transistor to turn on. The base-emitter junction in the transistor is just like a diode.
Base-Emitter voltage VBE = 0.7 V

9. Cont.….
The base current causes a collector current to flow IC = β x IB
Current flow through the collector resistor RC will cause a voltage drop on this resistor of VRC and the VOUT = VC will start to decrease from the value of VCC the supply voltage. This follows from KVL.
VCC - VRC - VC = 0 so VC = VCC - VRC

10. Example Calculate the values of IB, VBE, IC and VC. ANS…
IB = (30 V V)/ 1 MΩ = 30 uA.
IC = β x IB = 100x 30 uA= 3 mA.
Vrc= IC x RC = 3 mA x 5 kΩ = 15 V
VC = 30 V – 15V = 15 V

11. Amplifier Operation with a Bias Circuit

12. Operation
One Coupling capacitor is used between an as source and base . other coupling capacitor is used between collector and load.
Coupling capacitor act in dc circuit like open circuited and in ac circuit act like short circuit.
Main reason of use of coupling capacitor is prevent the ac source and load resistance changing the Q-point.
As shown in fig (b),the source voltage is 100uv.so coupling capacitor act as short circuited.
So ac source voltage appears between base and ground.
The ac source voltage produces ac current that is added into dc existing current.

13. Cont.… an ac component is superimposed on to dc component.
During the positive half cycle ac base current 30uA add to the dc base current . during negative half cycle ac base current 30uA subtract to the dc base current .
The ac base current produces an amplified variation in collector current because of the current gain.
As shown in fig (a) collector current has dc component of 3mA superimposed into ac collector current.so amplified collector current flow to the collector resistor.

14. Cont.…
Again ac component is superimposed into ac component the collector voltage is swinging above and below dc level +15 V.
And ac collector voltage is inverted to 180 degree to original collector voltage.

15. Diagrams
Base current.
Collector current
Collector voltage.

16. Voltage waveforms

17. Gains….
Voltage gain
the voltage gain is define by the ratio of output voltage to the input voltage.
Gain is denoted by A.
Av = V out/ V in.
Av = 50mV / 100uV.
= 500.

18. 2.Current gain
Current gain is define as the ratio of the collector current to the base current.
It is denoted by Ai
Ai = Ic/Ib.

19. Output voltage. V out = A*V in. Where A voltage amplification.
V out = 200*2mv
= 400mV.

20. Input voltage
the Av is given in the datasheet of particular transistor and (V out ) is given then we easily calculate the (Vin):
V in = V out /Av.
=2.5/350
=7.14mv.

21. THANK YOU…….