SUB.TEACHER:- MR.PRAVIN BARAD NAME:-SAGAR KUMBHANI ( ) -VIKRAMSINH JADAV( ) -PARECHA TUSHAR( ) TOPIC:-LINEAR AMPLIFIER(BJT AMPLIFIER) Analog Electronic(213009)

Understanding of concepts related to: Transistors as linear amplifiers dc and ac equivalent circuits Use of coupling and bypass capacitors to modify dc and ac equivalent circuits Small-signal voltages and currents Small-signal models for diodes and transistors Identification of common-emitter amplifiers Amplifier characteristics such as voltage gain, input and output resistances and linear signal range Rule-of-thumb estimates for voltage gain of common-emitter amplifiers.

The BJT is an an excellent amplifier when biased in the forward- active region. The FET can be used as an amplifier if operated in the saturation region. In these regions, the transistors can provide high voltage, current and power gains. DC bias is provided to stabilize the operating point in the desired operation region. The DC Q-point also determines – The small-signal parameters of the transistor – The voltage gain, input resistance, and output resistance – The maximum input and output signal amplitudes – The overall power consumption of the amplifier

The AC equivalent circuit is constructed by assuming that all capacitances have zero impedance at signal frequency and the AC voltage source is at ground. Assume that the DC Q-point has already been calculated.

Overall voltage gain from source v i to output voltage v o across R 3 is:

The BJT is biased in the forward active region by dc voltage sources V BE and V CC = 10 V. The DC Q-point is set at, (V CE, I C ) = (5 V, 1.5 mA) with I B = 15  A. Total base-emitter voltage is: Collector-emitter voltage is: This produces a load line.

Static power dissipation in amplifiers is determined from their DC equivalent circuits. Total power dissipated in C-B and E-B junctions is: where Total power supplied is: The difference is the power dissipated by the bias resistors.

An 8 mV peak change in v BE gives a 5  A change in i B and a 0.5 mA change in i C. The 0.5 mA change in i C gives a 1.65 V change in v CE. If changes in operating currents and voltages are small enough, then I C and V CE waveforms are undistorted replicas of the input signal. A small voltage change at the base causes a large voltage change at the collector. The voltage gain is given by: The minus sign indicates a phase shift between input and output signals.

AC coupling through capacitors is used to inject an ac input signal and extract the ac output signal without disturbing the DC Q-point Capacitors provide negligible impedance at frequencies of interest and provide open circuits at dc. In a practical amplifier design, C 1 and C 3 are large coupling capacitors or dc blocking capacitors, their reactance (X C = |Z C | = 1/  C) at signal frequency is negligible. They are effective open circuits for the circuit when DC bias is considered. C 2 is a bypass capacitor. It provides a low impedance path for ac current from emitter to ground. It effectively removes R E (required for good Q-point stability) from the circuit when ac signals are considered.

DC analysis: Find the DC equivalent circuit by replacing all capacitors by open circuits and inductors (if any) by short circuits. Find the DC Q-point from the equivalent circuit by using the appropriate large-signal transistor model. AC analysis: Find the AC equivalent circuit by replacing all capacitors by short circuits, inductors (if any) by open circuits, dc voltage sources by ground connections and dc current sources by open circuits. Replace the transistor by its small-signal model (to be developed). Use this equivalent circuit to analyze the AC characteristics of the amplifier. Combine the results of dc and ac analysis (superposition) to yield the total voltages and currents in the circuit.

All capacitors in the original amplifier circuit are replaced by open circuits, disconnecting v I, R I, and R 3 from the circuit and leaving R E intact. The the transistor Q will be replaced by its DC model. DC Equivalent Circuit

The coupling and bypass capacitors are replaced by short circuits. The DC voltage supplies are replaced with short circuits, which in this case connect to ground.

By combining parallel resistors into equivalent R B and R, the equivalent AC circuit above is constructed. Here, the transistor will be replaced by its equivalent small-signal AC model (to be developed).

For the present we consider DC behaviour and assume that we are working in the normal linear amplifier regime with the BE junction forward biased and the CB junction reverse biased

Treating the transistor as a current node: Also:

Define a common emitter current-transfer ratio  Such that:

Since reverse saturation current is negligible the second term on the right hand side of this equation can usually be neglected (even though (1- α) is small) Thus

A small change in α causes a much bigger change in ß which means that ß can vary significantly, even from transistor to transistor of the same type. We must try and allow for these variations in circuit design.

Hence: which after some rearrangement gives

Avoid this saturation region where we try to forward bias both junctions

We can therefore draw an input characteristic (plotting base current I B against base-emitter voltage V BE ) and an output characteristic (plotting collector current I c against collector-emitter voltage V CE )

Uses the same stabilised bias scheme as CE configuration. The base is grounded for a.c. signals using a large capacitor. Signal input at the emitter (between the emitter and the common base) Signal output at the collector (between the collector and the common base)

+V CC GND RERE R1R1 R2R2 CBCB v in RCRC v out RLRL

Input impedance to stage (whole amplifier)

Signal input between base and common collector. Signal output between emitter and common collector.

Transistor input impedance =

The voltage gain is slightly less than unity. The CC circuit is sometimes known as an emitter follower since the output voltage at the emitter follows the input voltage at the base.

The prime function of this circuit is to connect a high resistance source to a low resistance load (“a buffer amplifier”). Since voltage gain is approximately one this is achievable as the current gain is high.

35  = g m r 

The hybrid-pi small-signal model is the intrinsic low- frequency representation of the BJT. The small-signal parameters are controlled by the Q-point and are independent of the geometry of the BJT. Transconductance: Input resistance: Output resistance:

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