modified draft exp7 feedback amplifiers

EE321: Analog Circuits Laboratory

Experiment 7: Negative Feedback Amplifiers using BJT

Objectives:

1. To study the influence of the negative feedback in BJT amplifier circuits.2. To examine via experimentation the properties of the Current-Shunt, Voltage-Series and

Voltage-Shunt feedback BJT amplifiers. 3. To determine the input impedance, output impedance, gain, bandwidth of BJT amplifiers

with and without feedback.

Pre Lab Work:

Read about feedback Amplifiers from text book.

Introduction and Theory

Transistors amplifiers are commonly used in applications like RF (radio frequency), audio, OFC (optic fiber communication) etc. Anyway the most common application we see in our day to day life is the usage of transistor as an audio amplifier. As you know there are three transistor configurations that are used commonly i.e. common base (CB), common collector (CC) and common emitter (CE). In common base configuration has a gain less than unity and common collector configuration (emitter follower) has a gain almost equal to unity). Common emitter follower has a gain that is positive and greater than unity. So, common emitter configuration is most commonly used in audio amplifier applications.A good transistor amplifier must have the following parameters; high input impedance, high band width, high gain, high slew rate, high linearity, high efficiency, high stability etc.

Feedback plays a very important role in electronic circuits and the basic parameters, such as input impedance, output impedance, current and voltage gain and bandwidth, may be altered considerably by the use of feedback for a given amplifier. A portion of the output signal is taken from the output of the amplifier and is combined with the normal input signal and thereby the feedback is accomplished.

There are two types of feedback. They are i) Positive feedback and ii) Negative feedback.

Negative feedback helps to increase the bandwidth, decrease gain, distortion, and noise, modify input and output resistances as desired. An amplifier circuit equipped with some amount of negative feedback is not only more stable, but it distorts the input waveform less and is generally capable of amplifying a wider range of frequencies. The tradeoff for these advantages (there just has to be a disadvantage to negative feedback, right?) is decreased gain. If a portion of an amplifier's output signal is “fed back” to the input to oppose any changes in the output, it will require a greater input signal amplitude to drive the amplifier's output to the same amplitude as before. This constitutes a decreased gain. However, the advantages of stability, lower distortion, and greater bandwidth are worth the tradeoff in reduced gain for many applications.

1

A current shunt feedback amplifier circuit is illustrated in the figure 1. It is called a series-derived, shunt-fed feedback. The shunt connection at the input reduces the input resistance and the series connection at the output increases the output resistance. This is a true current amplifier.

Voltage shunt feedback is also called shunt derived shunt feedback connection. Here a fraction of the output is supplied in parallel with input voltage through the feedback network. This type of amplifier is also called as trans-resistance amplifier.

In Current-Series Feedback, the input impedance and the output impedance are increased. Noise and distortions are reduced considerably.

Figure 1 shows the basic feedback topologies. The effect of the feedback topology on the amplifier input-output resistance levels can be summarized as follows:

Fig 1. Basic feedback topologies

Table (i): The different topologies of the feedback with their analysis a) Current-shunt b) Voltage-shunt c) Current-series d) voltage series

2

Characteristics Voltage-series Current –series Current-shunt Voltage-shunt

Table (ii): The effect of Negative feedback on Amplifier characteristics

Characteristics Voltage-series Current –series Current-shunt Voltage-shuntDecreases Increases Increases Decreases

Increases Increases Decreases Decreases

Gain Decreases Decreases Decreases DecreasesBandwidth Increases Increases Increases IncreasesNon-linear Decreases Decreases Decreases Decreases

PROCEDURE

A. INPUT AND OUTPUT IMPEDANCE MEASUREMENT FOR ANY AMPLIFIER CIRCUIT

Fig. (a) Measuring input impedance

1. Connections are made as per circuit diagram shown in Fig. (a).2. Signal generator is set to provide a sine wave output at 1kHz. The amplitude of the input

signal should be adjusted so that the display on the oscilloscope is noise free (large enough) and distortion free (not too large) say 200mV. Because of the attenuator the net input applied to amplifier will be 0.1 times signal value i.e., 20 mV. The display on the

3

oscilloscope screen should be as large as is practical and set so that its amplitude and half its amplitude can be easily estimated.

3. The resistance at the amplifier input should then be increased until the output waveform is exactly half its previously set value. At this setting the signal is shared equally between the test resistance and the input impedance of the amplifier, meaning that the resistance and impedance are equal. After switching off and removing the test resistance, measuring the variable resistor with an Ohm meter gives the value equivalent to the input impedance of the amplifier.

Fig. (b) Measuring Output impedance

4. Connections are made as per circuit diagram shown in Fig. (b).5. The measurement of output impedance uses the same method as for input impedance but

with different connections. In this case the amplifier load is replaced with variable resistor. Care must be taken however, to ensure that the resistance connected in place of the load is able to dissipate sufficient power without damage

6. Initially the output from the amplifier should be adjusted for a display similar to that used for the input impedance test, but this time with no load connected to the output terminals. The test resistance is then connected across the output terminals and adjusted for maximum resistance before switching on the amplifier. The test resistance is reduced in value until the display indicates half the amplitude of that noted with no load. This test resistance is now the same value as the output impedance.

7. Now again measure the input and output impedance with feedback in the amplifier circuit.

B. VOLTAGE- SERIES FEEDBACK AMPLIFIER

4

200mVrms 1kHz 0° vi

Q1

BC 547

Rc1

4.7kΩ68kΩR1

2.2kΩRe22kΩ

R2

100ΩR5

Q2

BC 547

R5

2.2kΩ47kΩR1

33kΩR2

2.2kΩRe

VCC12V

10µFC2

C2

10µF

100µFCE

CE1100µF

10µFCB

20kΩR2

180kΩR2

1kΩRL

DSO

(a)


Q1

BC 547

Rc1

4.7kΩ68kΩR1

2.2kΩRe22kΩ

R2

100ΩR5

Q2

BC 547

R5

2.2kΩ47kΩR1

33kΩR2

2.2kΩRe

VCC12V

10µFC2

C2

10µF

100µFCE

CE1100µF

10µFCB

20kΩR2

180kΩR2

1kΩRL

DSO

4.7kΩRf

10µFC2

(b)

Fig.-2 Circuit diagram of Voltage-series feedback Amplifier (a) without feedback (b) with feedback

5

Case a: Without feed back

1. Connections are made as per circuit diagram Fig. 2(a)2. Measure input and output impedance of the amplifier as described in section III (a)3. Connect function generator to give sine wave input signal (V in) and set the value of Vin at

200 mV peak to peak and frequency 20 Hz.

4. To generate very small signal of the order of 20 mV pp, we used voltage divider (of voltage gain 1:10) between signal generator and amplifier input.

5. To view the output and the input signal simultaneously, connect one probe of CRO/DSO

to Vin and another probe across RL. Connect ground of DSO, DC power supply and

Function Generator very close to each other at a single point on breadboard and to

amplifier ground.

6. Keep the input voltage constant at 20mV peak-peak and 1 KHz frequency. Note down the

output voltage and calculate the gain by using the expression (This is the gain at mid-frequency which corresponds to maximum gain)

7. Keeping the input voltage at constant at 20mV peak-peak, the frequency is slowly

increased until output voltage becomes 0.707 . Stop and note down the frequency which corresponds to higher cut-off frequency.

8. Repeat the same procedure by decreasing the frequency and note down the frequency at

which output voltage becomes 0.707 , which corresponds to lower cut-off frequency.9. The Bandwidth of the amplifier is calculated from the graph using the expression

Bandwidth

.10. The gain-bandwidth product of the amplifier is calculated by using the expression

Gain-Bandwidth Product = (3dB mid-band gain) X (Bandwidth).

Case (b): With feed back

11. Connections are made as per circuit diagram Fig. 2(b)12. Measure input and output impedance as described in section III (a)13. Keep the input voltage constant at 20mV peak-peak and 1 KHz frequency. Note down the


6





Bandwidth



Note down the following:

PracticalWithout feedback With feedback

Input ImpedanceOutput ImpedanceGain (Mid Band) in dBLower cut-off frequency ( fL )Higher cut-off frequency ( fH )Band width ( fH—fL )Gain-Bandwidth Product

Fig. (c) Model Frequency response of a feedback amplifier with and without feedback

C. VOLTAGE- SHUNT FEEDBACK AMPLIFIER

7

Q1

BC 547

4.7kΩR468kΩ

R1

1.2kΩR5

15kΩR2

VCC12V

22µFC2

22µFCB


20kΩR2

180kΩR1

1kΩRL

DSO

47µFCE

(a)

Q1

BC 547

4.7kΩR468kΩ

R1

1.2kΩR5

15kΩR2

VCC12V

22µFC2

22µFCB


20kΩR2

180kΩR1

1kΩRL

DSO

47µFCE

22µFCf

47kΩRf

(b)Fig.-3 Circuit diagram Voltage-shunt feedback amplifier (a) without feedback (b) with

feedbackCase a: Without feed back

1. Connections are made as per circuit diagram Fig. 3(a)2. Measure input and output impedance as described in section III (a)

8

3. Keep the input voltage constant at 20mV peak-peak and 1 KHz frequency. Note down the






Bandwidth










Bandwidth





Input ImpedanceOutput ImpedanceGain (Mid Band) in dBLower cut-off frequency ( fL )

9

Higher cut-off frequency ( fH )Band width ( fH—fL )Gain-Bandwidth Product

D. CURRENT- SHUNT FEEDBACK AMPLIFIER

Q1

BC 547

Rc1

10kΩ47kΩR1

5kΩR2

2kΩRe1

Q2

BC 547

47kΩRc2

47kΩR3

5kΩR4

2kΩRe2

VCC12V

1kΩRL

22µFCB


20kΩR2

180kΩR1

DSO

22µFCB

22µFCB

100µFCB

(a)

10

Q1

BC 547

Rc1

10kΩ47kΩR1

5kΩR2

2kΩRe1

Q2

BC 547

47kΩRc2

47kΩR3

5kΩR4

2kΩRe2

VCC12V

1kΩRL

22µFCB


20kΩR2

180kΩR1

DSO

22µFCB

22µFCB

100µFCB

47kΩRf

S1

Key = Space 22µFCf

(b)Fig.-4 Circuit diagram Current-shunt feedback amplifier (a) without feedback (b) with

feedback

Case a: Without feed back

1. Connections are made as per circuit diagram Fig. 4(a)2. Measure input and output impedance as described in section III (a)3. Keep the input voltage constant at 20mV peak-peak and 1 KHz frequency. Note down the






Bandwidth




11







Bandwidth





Input ImpedanceOutput ImpedanceGain (Mid Band) in dBLower cut-off frequency ( fL )Higher cut-off frequency ( fH )Band width ( fH—fL )Gain-Bandwidth Product

---------------------------------------------------------------------------

12

modified draft exp7 feedback amplifiers

Documents

voltage shunt feedback

negative feedback amplifiers

positive feedback

use of feedback

feedback network

types of feedback

high gain

input voltage