Archive for the ‘S.Y.B.Sc.(Physics)’ Category
As shown in above circuit, PNP transistor is connected with emitter-base PN junction as forward bias by VEE whereas collector-base diode is reverse-biased by Vcc. AC signal is given by the source Vs through resistance Rs and output voltage is obtained across collector and base as voltage Vo’.
Two-Diode Analogy of CB Amplifier:
PNP Two-Diode Analogy of CB Amplifier
As shown above, PNP transistor is connected as two-diode analog. Both the diodes are connected in front-to-front position, since arrow on the emitter points inward.
Operation of Circuit:
With no input AC signal, the emitter current is DC bias current IE. Most of this current is swept into collector current of the transistor as base current IB is small. i.e. IE=IB+IC…………(1) In presence of input AC signal Vs, there will be DC as well as current in the circuit and therefor above equation becomes iE=iB+iC
The above graph of IE —–>VEB gives input characteristics of CB amplifier. Since diode is in forward-biased, this characteristic is similar to that of diode in forward bias. But input or controlling current is much larger than in the CE amplifier.
The above graph of Ic——>VCB for different values of IE gives output characteristics of a CB amplifier. Due to high output resistance, the output characteristics curve is almost horizontal.
Since base is narrow and collector is heavily doped with respect to the base, current can flow in the collector circuit though collector-base diode is reverse bias. The amount of collector current depends on how well base-emitter junction is forward bias. Thus large the forward bias VBE and the base current IB, larger will be Ic for a given Vcc.
Hence output or collector characteristics curve is drawn for Ic—->VcE for different values of IB
(Click on the following image for more explanation)
As shown in above circuit transistor consist of two PN junctions, so that the transistor can be thought of as two diodes- in this case back-to-back position. Diode point outward as arrow on the emitter lead of the NPN transistor.
In normal amplifier, as shown in above circuit the base-emitter junction is always forward bias by VBB and collector-base junction is always reverse bias by Vcc. Moreover, since VCE=VCB+VBE and VBE is very small then VCB=VCE
This diagram is used to emphasize the correct biasing of the transistor.
Transistor Input Characteristics:
As shown in above diagram two diode analogy is used to explain the input characteristic of a transistor. Since base-emitter junction is forward bias, VBE is very small and 0.6V for germanium transistor and 0.2V for silicon transistor.
The iB—–>VBE curve gives input characteristics for different values of output voltage VCE of transistor. From this graph we can say, as voltage between base and emitter increase (for the voltage greater than 0.6V for germanium) the base current iB increases sharply for different values of output voltage VCE. So what happen at the input depends upon the output voltage VCE We get different curve for different values of output voltage VCE. This difference is due to change in depletion region under reverse bias condition.
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As shown in the above circuit diagram, the input ac signal Vs is given through resistance Rs to the base of transistor. Supply voltage VBB is give between base and emitter of transistor, which keeps the transistor in the center of its characteristics. At the output, Vcc supply is given through load resistance RL to the collector of transistor. The voltage between base and emitter is known as VBE and that of between collector and emitter is called VCE. It should also remember that VBB and VCC supply a dc current whereas Vs supply ac signal.
Initially when no input signal Vs, VBB will supply dc current into base IB so that corresponding dc current Ic will flow. The sum of two current IB and Ic will turn as emitter current IE i.e IE=Ic+IB. This is the condition when no input signal is applied.
When input signal Vs is applied to the base of transistor, corresponding ac current is will flow in the base which will be added to IB resulting in an ac component of current riding on dc called iB. Which in turn will give ac component of current ic to flow in the collector and this will be added to dc Ic.
The doping of junctions and width of base is so chosen that small base current iB will give large collector current ic. Therefore transistor has capability of current gain Ai. Since voltage from base to emitter is small and voltage in the collector circuit is large voltage gain Av is also obtained.
So for a transistor, iE=ic+iB and VCE=VCB+VBE
Comparison o f CE Amplifier with Triod Valve:
The main difference between transistor and triode is that transistor is a current controlled device whereas triode is a voltage controlled device.
Working of Basic NPN common emitter amplifier:
(Click on the following image for working of CE amplifier and two diode analogy of a transistor)