Este documento contiene una lista de problemas donde se calcula el punto Q de operación de transistor BJT, así como la ganancia en pequeña señal e impedancia de entrada/salida.Descripción completa
Full description
bjt
Descripción completa
praktikumFull description
laboratorio uis transistor Bjt como interruptorDescripción completa
[sfgbhasfFull description
Descripción: Informe configuraciones básicas bjt Descripción de la polarización y modelos de pequeña seññal.
Descripción: Una breve investigación sobre los amplificadores BJT y las clases de amplificadores
Descripción completa
PACIFIC INSTITUTE 0F TECHNOLOGY
EXPERIMENT NO.1 AIM
Plot gain-frequency characteristics of BJT amplifier with and without negative feedback in the emitter circuit and determine bandwidths, gain bandwidth products and gains at 1kHz with and without negative feedback. APPARATUS REQUIRED
Feedback amplifier kit board, cathode ray oscilloscope, function generator, connecting probes and leads. THEORY
When a part or fraction of output is combined back to the inputs, feedback is said to exit. Thus the process of combining a fraction of output energy back to the input is called the feedback. There are two type of feedback. Positive feedback: - When the feedback signal is in phase with input signal, it is called positive
or regenerative feedback. It is used in oscillator circuit. Negative feedback: - When the feedback signal is in phase opposition to the input signal, it is
called negative or degenerative feedback. It is used in amplifier circuit. Negative feedback in amplifiers: - Negative feedback can reduce the gain of the amplifier, but
it has many advantages, such as stabilization of gain, reduction of nonlinear distortion and noise, control of input and output impedances, and extension of bandwidth.
Fig 1.1: Block diagram of feedback amplifier
Gain of amplifier with feedback Avf is is given by expression
DEPARTMENT OF ELECTRONICS AND COMMUNICATION
Page 1
PACIFIC INSTITUTE 0F TECHNOLOGY Avf =
Av/1+ß Av
Where Av= Gain of amplifier without feedback ß = feedback coefficient If ß Av>>1 then the expression becomes Avf = 1/ß So that overall gain of feedback amplifier Avf is independent of internal gain and depends only on feedback coefficient ß and ß in turn depends on the passive elements such as resistors which remains fairly constant so gain is stabilized. Types of negative feedback in Amplifier: -
If the feedback network samples the output voltage, it is voltage feedback. If it samples the output current, it is current feedback. The feedback signal can be connected in series or in parallel with the signal source and the amplifier input terminals, so called series feedback and parallel feedback . So there are four types of negative feedback in amplifier. 1. Voltage Series feedback 2. Current Series feedback 3. Voltage Shunt feedback 4. Current Shunt feedback
Fig. 1.2: Voltage shunt feedback amplifier
Fig shows the voltage shunt feedback in which resistor R f provide feedback from collector to base of a CE amplifier. The feedback current is given by: If = Vin – Vout/ R f Since Vin << Vout , so If = – Vout/ R f If = -ß Vout
where ß =1/ R f
The most advantage of the negative feedback is that by proper use of this, there is DEPARTMENT OF ELECTRONICS AND COMMUNICATION
Page 2
PACIFIC INSTITUTE 0F TECHNOLOGY significant improvement in the frequency response and in the linearity of the operation of the amplifier. This disadvantage of the negative feedback is that the voltage gain is decreased. PRACTICAL CIRCUIT DAIGRAM
Fig 1.3: Kit diagram of voltage shunt feedback amplifier
PROCEDURE 1. Make connections as per circuit diagram. 2. Connect a 12V supply to the circuit. 3. For plotting the frequency the input voltage is kept constant at 100mV peak-peak from
function generator and the frequency is varied from 100Hz to 1MHz. 4. Note down the output voltage on CRO for each frequency. 5.
Press the switch and repeat steps 3 and 4 again.
6. Tabulate all the readings and calculated the voltage gain in dB by using
expression Av = 20log (V0 / Vi ) dB 7.
Draw a graph by taking frequency on X-axis and gain on Y-axis on semi log graph.
8. Calculate the Bandwidth of the amplifier from the graph using the expression
Bandwidth B.W = f 2 – f 1. Where f 1 is lower cutoff frequency and f 2 is upper cutoff frequency of CE amplifier. 9. Calculate the gain-bandwidth product of the amplifier by using the expression
Gain-Bandwidth Product = 3-dB mid band gain X Bandwidth. DEPARTMENT OF ELECTRONICS AND COMMUNICATION
Page 3
PACIFIC INSTITUTE 0F TECHNOLOGY
OBSERVATION TABLE
Vi = ………………… S.No
Frequency With Feed Back In Output Voltage Hz voltage gain Avf =V0 /Vi Vo
With Out Feed Back Gain in dB Output Voltage = 20logAvf voltage gain Vo Av=V0/Vi
Gain in dB =20logAv
CALCULATION
1. Calculate the voltage gain in dB for all output voltage by using expression Av = 20log (V0 / Vi ) dB 2. Plot the frequency response of CE amplifier with feedback and without feedback and identify maximum gain region for both. 3. Calculate -3dB frequencies for both by draw (max. gain in dB -3dB ) line on both the graph. 4.
Calculate bandwidth by using expression Bandwidth B.W = f 2 – f 1.
5. Calculate the gain-bandwidth product of the amplifier by using the expression Gain-Bandwidth Product = 3-dB mid band gain X Bandwidth.
DEPARTMENT OF ELECTRONICS AND COMMUNICATION
Page 4
PACIFIC INSTITUTE 0F TECHNOLOGY MODEL GRAPH
Fig1.4: Graph between frequency and gain
PRECAUTIONS
1 While taking the observations for the frequency response, the input voltage must be maintained constant at 20mV. 2 The frequency should be slowly increased in steps. 3 The three terminals of the transistor should be carefully identified. 4 All the connections should be correct. RESULT
The effect of negative feedback (Voltage -Shunt Feedback) on the amplifier is observed and we have plotted gain frequency characteristics of CE amplifier with and without negative feedback. Also determined following parameters: Parameter
With Feedback
Without Feedback
Gain Bandwidth GainBandwidth product
DEPARTMENT OF ELECTRONICS AND COMMUNICATION
Page 5
PACIFIC INSTITUTE 0F TECHNOLOGY
SAMPLE READING TABLE
Vi = 100mV S.No
1. 2.
Frequency With Feed Back In Output Voltage Hz voltage gain Vo Avf =V0 /Vi 100 .08V 500K .07V
With Out Feed Back Gain in dB Output Voltage Gain in dB = 20logAvf voltage gain =20logAv Vo Av=V0/Vi .3V .14V
CONCLUSION
_ _ _ _ VIVA QUESTION
1. What is meant by Feedback? 2. What are the types of feedback amplifiers? Ex plain? 3. Draw the circuit for voltage series feedback? 4. What are the differences between positive and ne gative feedback? 5. What is the effect of negative feedback on gain of an amplifier? 6. What is the formula for voltage gain with negative feedback? 7. What are the other names for positive and negative feedback circuits? 8. What is the formula for input resistance of a voltage series feedback? 9. What is the formula for output resistance of a voltage series feedback?
