class a power amplifier
THEORY:
The function of power amplifier is to raise the power level of input signal. Class A power amplifier is one in which the output current flows during the entire cycle of input signal. Thus the operating point is selected in such away that the transistor operates only over the linear region of its load line. So this amplifier can amplify input signal of small amplitude. As the transistor operates over the linear portion of load line the output wave form is exactly similar to the input wave form. Hence this amplifier is used where freedom from distortion is the prime aim.
8. CLASS B COMPLEMENTARY SYMMETRY PUSH PULL AMPLIFIER
THEORY:
Complementary means the circuit uses two identical transistors but one is NPN and other is PNP. The symmetry means the biasing resistors connected in both transistors are equal. As a result of this, emitter base junction of each transistor is biased with the same voltage.
During the positive half cycle of ac input the base emitter voltage of both transistors becomes positive. Under this condition only NPN transistor conducts, while PNP transistor is cutoff. During this process positive half cycle current flows through load resistor R5.
2.COMMON SOURCE AMPLIFIER
THEORY:
A weak signal is applied between gate and source and output is obtained at drain. For the proper operation of FET, gate must be reverse biased. A small change in reverse bias on the gate produces a large drain current. This fact makes FET capable of raising the strength of a weak signal. The gain of the common source FET amplifier is very high which is greater than unity.
3. RC COUPLED AMPLIFIER
THEORY:
RC is the most widely used coupling as it provides excellent audio fidelity. A coupling capacitor is used to connect output of first stage to the input of the second stage. Resistances R1, R2, R5, R3, R4 and R10 form biasing and stabilisation network. Emitter bypass capacitors C5 and C6 offer low reactance paths to the signals. The coupling capacitor C3 transmits ac signal and blocks dc signal. Cascaded stages amplifies signal and overall gain is improved. The total gain is less than the product of gains of individual stages. Thus overall gain of two stages is A = A1 x A2, where A1 = Voltage gain of first stage and A2 = Voltage gain of second stage
When ac signal is applied to the base of the transistor Q1, its amplified output appears across the collector resistor R9. It is given to the second stage for further amplification and signal appears with more strength. Frequency response curve is obtained by plotting a graph between frequency and gain in dB. The gain is constant in midband frequency range and gain decreases in low and high frequency ranges. The gain decreases in the low frequency range due to coupling capacitor C3 and at high frequencies due to junction capacitance Cbe .
4. RC PHASE SHIFT OSCILLATOR
RC-Phase shift Oscillator has a CE amplifier followed by three sections of RC phase shift feed back networks. The out put of the last stage is return to the input of the amplifier. The values of R and C are chosen such that the phase shift of each RC section is 60º. Thus The RC ladder network produces a total phase shift of 180º between its input and output voltage for the given frequencies. Since CE Amplifier produces 180 º phases shift the total phase shift from the base of the transistor around the circuit and back to the base will be exactly 360º or 0º. This satisfies the Barkhausen condition for sustaining oscillations and total loop gain of this circuit is greater than or equal to 1, this condition used to generate the sinusoidal oscillations.
The frequency of oscillations of RC-Phase Shift Oscillator is,
f =1/2∏RC* √6
5. SINGLE TUNED VOLTAGE AMPLIFIER
THEORY:
The signal to be amplified is applied between the terminals base and emitter. The tank circuit is tuned (i.e L or C may be varied) in such a way that the resonant frequency becomes equal to the frequency of the input signal. At resonance the tuned circuit offers very high impedance and thus, the given input signal is amplified by the amplifier and appears with large value across it and other frequencies will be rejected. So the tuned circuit selects the derived frequency and rejects all other frequencies.
6. SERIES VOLTAGE REGULATOR
THEORY:
Voltage regulator converts a dc input voltage in to a chosen dc voltage which is stable under conditions of load current and input variation. A series regulator using an additional transistor as an error amplifier, it improves the line and load regulation of the circuit. Resistor R2 and zener diode are the reference source. Transistor Q2 and its associated circuit components constitute the error amplifier, that controls the series pass transistor. When the circuit output changes, the change is amplified by transistor Q2 and fed back to the base of Q1 to correct the output voltage level. Now suppose Vo decreases , VBE2 decreases .Because emitter voltage of Q2 is held at Vz, any decrease in VBE2 appears across the base emitter of Q2.A reduction ib VBE2 causes IC2 to be reduced,VR1 is reduced and VB1 is increased causing the output voltage increase.
enjoy!($!%#u)
During negative half cycle of ac input only PNP transistor conducts and NPN transistor is cutoff and the negative half cycle current flows through R5. We get a complete amplified wave form of input signal. This amplifier circuit has a unity gain because of the emitter follower configuration is used