Multistage Transistor Amplifier

-Define Multistage Transistor Amplifier?

-Write down the names of multistage transistor amplifier?

Mohammad Shakirul Islam 151-15-311

MULTISTAGE TRANSISTOR AMPLIFIER

A transistor circuit containing more than one stage of amplification is known as multistage transistor amplifier.

Fig: Block diagram of a 3-stage amplifier In a multistage amplifier, a number of single amplifiers are connected.

THE NAMES OF MULTISTAGE TRANSISTOR AMPLIFIER?

a) R-C coupled amplifier

b) Transformer coupled amplifier

c) Direct coupled amplifier

(i) In RC coupling, a capacitor is used as the coupling device. The capacitor connects the output of one stage to the input of the next stage in order to pass the a.c. signal on while blocking the d.c. bias voltages.

(ii) In transformer coupling, transformer is used as the coupling device. The transformer coupling provides the same two functions but permits in addition impedance matching.

(iii) In direct coupling or d.c. coupling, the individual amplifier stage bias conditions are so designed that the two stages may be directly connected without the necessity for d.c. isolation.

THE NAMES OF MULTISTAGE TRANSISTOR AMPLIFIER?

Jannatul Ferdous Id: 151-15-4888

WHAT IS FEEDBACK?

The process of injecting a fraction of output energy of device back to the input is known as feedback.

Principles of Electronic Feedback: 1. Positive Feedback.

2. Negative Feedback.

SHAMMI AKTER

Define Rc coupled transistor amplifier & mention why transformer coupling is used?

ID NO : 151-15-4970

RC Coupled Trasistor Amplifier & Transformer Coupled Amplifier edback

RC COUPLED TRANSISTOR AMPLIFIER

TRASFORMER-COUPLED AMPLIFIER

WE USE IT BECAUSE.....

No signal power is lost in the collector or base resistor. An excellent impedance matching can be achieved in a transformer coupled amplifier. Due to excellent impedance matching, it provides higher gain.

H.M TASFIK KIBRIA151-15-4985

Positive feedback: When the feedback energy (voltage or current) is in phase with the input signal and thus aids it, it is called positive feedback.

In this Figure, both amplifier and feedback network introduce a phase shift of 180°. The result is a 360° phase shift around the loop, causing the feedback voltage Vf to be in phase with the input signal Vin.

Negative feedback: When the feedback energy (voltage or current) is out of phase with the input signal and thus opposes it, it is called negative feedback.

In This figure, As we can see, the amplifier introduces a phase shift of 180° into the circuit while the feedback network is so designed that it introduces no phase shift (i.e., 0° phase shift). The result is that the feedback voltage Vf is 180° out of phase with the input signal Vin.

Hasanul Ferdous151-15-5197

Short Notes

Gain: The ratio of the output electrical quantity to the input one of the amplifier is called its gain.

it can be current gain or voltage gain or power gain.

The gain of a multistage amplifier is equal to the product of gains of individual stages. For instance, if G1, G2 and G3 are the individual voltage gains of a three-stage amplifier, then total voltage gain G is given by :

G = G1 × G2 × G3

Frequency response: The voltage gain of an amplifier varies with signal frequency. It is because reactance of the capacitors in the circuit changes with signal frequency and hence affects the output voltage.The curve between voltage gain and signal frequency of an amplifier is known as frequency response. The gain of the amplifier increases as the frequency increases from zero till it becomes maximum at fr, called resonant frequency. If the frequency of signal increases beyond fr, the gain decreases.

Decibel Gain: Although the gain of an amplifier can be expressed as a number, yet it is of great practical importance to assign it a unit. The unit assigned is bel or decibel (db). The common logarithm (log to the base 10) of power gain is known as bel power gain i.e.

Power Gain= log10 (Pout/Pin ) bel

1 bel = 10 dbIf the two powers are developed in the same resistance or equal resistances, then,

P1 = (Vin2 / R) = Iin

2 R

Pout = (Vout2 / R) = Iout

2 R

Bandwidth: The range of frequency over which the voltage gain is equal to or greater than 70.7% of the maximum gain is known as bandwidth.

The voltage gain of an amplifier changes with frequency. Referring to the frequency response in Fig, it is clear that for any frequency lying between f1 and f2 the gain is equal to or greater than 70.7% of the maximum gain. Therefore, (f1 - f2 ) is the bandwidth.

SADEKA HAQUE ID-151-15-5210

NEGATIVE FEEDBACKDefine Negative Feedback & also mention the advantage of Negative feedback.

What is Negative Feedback ???

Negative feedback. When the feedback energy (voltage or current) is out of phase with the input signal and thus opposes it, it is called negative feedback.

Fig : Negative Feedback

1. GAIN STABILITY2. NOISE REDUCTION3. REDUCTION IN NONLINEAR DISTORTION4. BANDWIDTH CAN BE INCREASED5. INCREASE IN INPUT IMPEDENCE6. DECREASE IN OUTPUT IMPEDENCE

Advantage Of Negative Feedback Advantages of Negative Feedback

Md. Ferdouse Ahmed Foysal ID-151-15-5274

PRINCIPLES OF NEGATIVE FEEDBACK

1. When negative voltage feedback is applied, the gain of the amplifier is reduced. Thus, the gain of above amplifier without feedback is 10,000 whereas with negative feedback, it is only 100.

Fig: Principles of negative voltage feedback in an amplifier.

Fig: Principles of negative voltage feedback in an amplifier.

2. When negative voltage feedback is employed, the voltage actually applied to the amplifier is extremely small. In this case, the signal voltage is 101 mV and the negative feedback is 100 mV so that voltage applied at the input of the amplifier is only 1 mV.

Fig: Principles of negative voltage feedback in an amplifier.

3. n a negative voltage feedback circuit, the feedback fraction m is always between 0 and 1.

Fig: Principles of negative voltage feedback in an amplifier.

The gain with feedback is sometimes called closed-loop gain while the gain without feedback is called open-loop gain. These terms come from the fact that amplifier and feedback circuits form a “loop”. When the loop is “opened” by disconnecting the feedback circuit from the input, the amplifier's gain is Av , the “open-loop” gain. When the loop is “closed” by connecting the feedback circuit, the gain decreases to Avf , the “closed-loop” gain.

Shahariar Rabby151-15-5424

VOLTAGE GAIN WITH NEGATIVE FEEDBACK

Here,Av = Gain;Avf = Gain with Feedback;eg = Signal Voltage;

e0 = Output Voltage;mv = Fraction Of Output Voltage Feedback;

The output e0 must be equal to the input voltage eg − mv e0 multiplied by gain Av of the amplifier i.e.,

But e0 / eg is the voltage gain of the amplifier with feedback.

∴ Voltage gain with negative feedback is

It may be seen that the gain of the amplifier

without feedback is Av. However, when negative

voltage feedback is applied, the gain is

reduced by a factor 1 + Av mv. It may be noted that negative voltage feedback does not affect the current

gain of the circuit.