Basic Electronics 4 by Dr. Mathivanan Velumani

BASIC ELECTRONICS- IV

Dr. V. Mathivanan

Associate Professor of Physics

E.mail: [email protected]

Multistage Transistor Amplifier

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

The name of the amplifier is usually given after the type of coupling used. e.g.

The capacitors serve the following two roles in transistor amplifiers :1. As coupling capacitors2. As bypass capacitors

1. As coupling capacitors.

(i) It blocks d.c. i.e. it provides d.c. isolation between the two stages of a multistage amplifier.

(ii) It passes the a.c. signal from one stage to the next with little or no distortion.

2. As bypass capacitors.

i. A bypass capacitor also blocks d.c.

ii. A bypass capacitor is connected in parallel with a circuit component (e.g. resistor) to bypass the a.c. signal and hence the name. A bypass capacitor CE is connected across the emitter resistance RE. Since CE behaves as a short to the a.c. signal, the whole of a.c. signal (ie) passes through it.

Important Terms

(i) Gain. The ratio of the output *electrical quantity to

the input one of the amplifier is called its gain.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

It is worthwhile to mention here that in practice, total gain G is less than G1 × G2 × G3 due to the loading effect of next stages.

(ii) 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.

When the gains are expressed in db, the overall gain of a multistage amplifier is the sum of gains of individual stages in db.

(iv) 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.

From the figure, f1 to f2 is called bandwidth

Example Express the following gains as a number :(i) Power gain of 40 db (ii) Power gain of 43 dbSolution.(i) Power gain = 40 db = 4 belIf we want to find the gain as a number, we should work from logarithm back to the original number. Gain = Antilog 4 = 104 = 10,000(ii) Power gain = 43 db = 4.3 bel

∴ Power gain = Antilog 4.3 = 2 × 104 = 20,000

Example . A three-stage amplifier has a first stage voltage gain of 100, second stagevoltage gain of 200 and third stage voltage gain of 400. Find the total voltage gain in db .Solution.First-stage voltage gain in db = 20 log10 100 = 20 × 2 = 40Second-stage voltage gain in db = 20 log10 200 = 20 × 2.3 = 46Third-stage voltage gain in db = 20 log10 400 = 20 × 2.6 = 52

∴ Total voltage gain = 40 + 46 + 52 = 138 db

Example A multistage amplifier employs five stages each of which has a power gain of 30. What is the total gain of the amplifier in db ?

(ii) If a negative feedback of 10 db is employed, find the resultant gain. Solution. Absolute gain of each stage = 30 No. of stages = 5

(i) Power gain of one stage in db = 10 log10 30 = 14.77 ∴ Total power gain = 5 × 14.77 = 73.85 db

(ii) Resultant power gain with negative feedback = 73.85 − 10 = 63.85 db

Example. A certain amplifier has voltage gain of 15 db. If the input signal voltage is 0.8V,what is the output voltage ?Solution.db voltage gain = 20 log10 V2/V1

or 15 = 20 log10 V2/V1

or 15/20 = log10 V2/V1

or 0.75 = log10 V2/0.8Taking antilogs, we get,Antilog 0.75 = Antilog (log10 V2/0.8)or 100.75 = V2/0.8

∴ V2 = 100.75 × 0.8 = 4.5 V

Example. In an amplifier, the maximum voltage gain is 2000 and occurs at 2 kHz. It falls to 1414 at 10 kHz and 50 Hz. Find :(i) Bandwidth (ii) Lower cut-off frequency (iii) Upper cut-off frequency.

RC Coupled Transistor Amplifier

Where: XC is the Capacitive Reactance in Ohms, ƒ is the frequency in Hertz and C is the AC capacitance in Farads, symbol F.

This is the most popular type of coupling because it is cheap and provides excellent audio fidelity over a wide range of frequency.

It is usually employed for voltage amplification. Fig. shows two stages of an RC

coupled amplifier. A coupling capacitor CC is used to connect the output of first stage to the base (i.e. input) of the second stage and so on. As the coupling from one stage to next is achieved by a coupling capacitor followed by a connection to a shunt resistor, therefore, such amplifiers are called resistance – capacitance coupled amplifiers.

The resistances R1, R2 and RE form the biasing and stabilization network.

The emitter bypass capacitor offers low reactance path to the signal. Without it, the voltage gain of each stage would be lost.

The coupling capacitor CC transmits a.c. signal but blocks d.c. This prevents d.c. interference between various stages and the shifting of operating point.

Operation. When a.c. signal is applied to the base of the first transistor, it appears in the amplified form across its collector load RC. The amplified signal developed across RC is given to base of next stage through coupling capacitor CC.

The second stage does further amplification of the signal. In this way, the cascaded (one after another) stages amplify the signal and the overall gain is considerably increased.

It may be mentioned here that total gain is less than the product of the gains of individual stages.

It is because when a second stage is made to follow the first stage, the effective load resistance of first stage is reduced due to the shunting effect of the input resistance of second stage.

Frequency response.

(i) At low frequencies (< 50 Hz), the reactance of coupling capacitor CC is quite high and hence very small part of signal will pass from one stage to the next stage.

Moreover, CE cannot shunt the emitter resistance RE effectively because of its large reactance at low frequencies.

These two factors cause a falling of voltage gain at low frequencies.

(ii) At high frequencies (> 20 kHz), the reactance of CC is very small and it behaves as a short circuit. This increases the loading effect of next stage and serves to reduce the voltage gain. Moreover, at high frequency, capacitive reactance of base- emitter junction is low which increases the base current. This reduces the current amplification factor β. Due to these two reasons, the voltage gain drops off at high frequency.

(iii) At mid-frequencies (50 Hz to 20 kHz), the voltage gain of the amplifier is constant. The effect of coupling capacitor in this frequency range is such so as to maintain a uniform voltage gain.

Thus, as the frequency increases in this range, reactance of CC decreases which tends to increase the gain.

However, at the same time, lower reactance means higher loading of first stage and hence lower gain.

These two factors almost cancel each other, resulting in a uniform gain at mid-frequency.

Transformer-Coupled AmplifierThe main reason for low voltage and power gain of RC coupled amplifier is that the effective load (RAC) of each stage is *decreased due to the low resistance presented by the input of each stage to the preceding stage. If the effective load resistance of each stage could be increased, the voltage and power gain could be increased. This can be achieved by transformer coupling.

Operation. When an a.c. signal is applied to the base of first transistor, it appears in the amplified form across primary P of the coupling transformer. The voltage developed across primary is transferred to the input of the next stage by the transformer secondary as shown in Fig. The second stage renders amplification in an exactly similar manner.

Frequency response. The frequency response of a transformer coupled amplifier is shown in Fig. It is clear that frequency response is rather poor i.e. gain is constant only over a small range of frequency. The output voltage is equal to the collector current multiplied by reactance of primary.

At low frequencies, the reactance of primary begins to fall, resulting in decreased gain. At high frequencies, the capacitance between turns of windings acts as a bypass condenser to reduce the output voltage and hence gain.

It follows, therefore, that there will be disproportionate amplification of frequencies in a complete signal such as music, speech etc. Hence, transformer-coupled amplifier introduces frequency distortion.

Advantages(i) No signal power is lost in the collector or base resistors.(ii) An excellent impedance matching can be achieved in a transformer coupled amplifier. It is easy to make the inductive reactance of primary equal to the output impedance of the transistor and inductive reactance of

secondary equal to the input impedance of next stage.(iii) Due to excellent impedance matching, transformer coupling provides higher gain.

Disadvantages(i) It has a poor frequency response i.e.the gain varies considerably with frequency.(ii) The coupling transformers are bulky and fairly expensive at audio frequencies.(iii) Frequency distortion is higher i.e. low frequency signals are

less amplified as compared to the high frequency signals.(iv) Transformer coupling tends to introduce *hum in the output.Applications. Transformer coupling is mostly employed for impedance matching. In general, the last stage of a multistage amplifier is the power stage. Here, a concentrated effort is made to transfer maximum power to the output device e.g. a loudspeaker.

FeedbackThe process of injecting a fraction of output

energy of some device back to the input is known as feedback.

It has been found very useful in reducing noise in amplifiers and making amplifier operation stable.

(i) 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. This is illustrated in Fig. 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.

The positive feedback increases the gain of the amplifier.

However, it has the disadvantages of increased distortion and instability.

Therefore, positive feedback is seldom employed in amplifiers.

One important use of positive feedback is in oscillators.

(ii) 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. This is illustrated in Fig. As you 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.

Negative feedback reduces the gain of the

amplifier.

However, the advantages of negative feedback are: reduction in distortion, stability in gain, increased bandwidth and

improved input and output impedances. It is due to these advantages that negative

feedback is frequently employed in amplifiers.

Gain of Negative Voltage Feedback Amplifier

Advantages of Negative Voltage Feedback

Emitter Follower

It is a negative voltage feedback circuit. The emitter follower is a current amplifier that has no voltage gain. Its most important characteristic is that it has high input impedance and low output impedance. This makes it an ideal circuit for impedance matching.

(i) There is neither collector resistor in the circuit nor there is emitter bypass capacitor. These are the two circuit recognition features of the emitter follower.(ii) Since the collector is at ac ground, this circuit is also known as common collector (CC) amplifier.

Operation. The input voltage is applied between base and emitter and the resulting a.c. emittercurrent produces an output voltage ieRE across the emitter resistance. This voltage opposes the input voltage, thus providing negative feedback. Clearly, it is a negative current feedback circuit since the voltage fedback is proportional to the emitter current i.e., output current. It is called emitter followerbecause the output voltage follows the input voltage.

Characteristics. The major characteristics of the emitter follower are :(i) No voltage gain. In fact, the voltage gain of an emitter follower is close to 1.(ii) Relatively high current gain and power gain.(iii) High input impedance and low output impedance.(iv) Input and output ac voltages are in phase.

Applications of Emitter Follower

(i) To provide current amplification with no voltage gain.(ii) Impedance matching.(i) Current amplification without voltage gain. We know that an emitter follower is a current amplifier that has no voltage gain (Aν j 1). There are many instances (especially in digital electronics) where an increase in current is required but no increase in voltage is needed. In such a situation,

an emitter follower can be used. For example, consider the two stage amplifier circuit as shown in Fig. Suppose this 2-stage amplifier has the desired voltage gain but current gain of this multistage amplifier is insufficient. In that case, we can use an emitter follower to increase the current gain without increasing the voltage gain.

(ii) Impedance matching. We know that an emitter follower has high input impedance and low output impedance. This makes the emitter follower an ideal circuit for impedance matching. Fig. shows the impedance matching by an emitter follower. Here the output impedance of the source is 120 kΩ while that of load is 20 Ω. The emitter follower has an input impedance of 120 kΩ and output impedance of 22 Ω. It is connected between high-impedance source and low impedance load. The net result of this arrangement is that maximum power is transferred from the original source to the original load. When an emitter follower is used for this purpose, it is called a bufferamplifier.

Hartley Oscillator

The tank circuit is made up of L1, L2 and C. The frequency of oscillations is determined by the values of L1, L2 and C and is given by :

Circuit operation. When the circuit is turned on, the capacitor is charged. When this capacitor is fully charged, it discharges through coils L1 and L2 setting up oscillations of frequency determined by *exp. (i). The output voltage of the amplifier appears across L1 and feedback voltage across L2. The voltage across L2 is 180° out of phase with the voltage developed across L1 (Vout) as shown in Fig. . It is easy to see that voltage fedback (i.e., voltage across L2) to the transistor provides positive feedback. A phase shift of 180° is produced by the transistor and a further phase shift of 180° is produced by L1 − L2 voltage divider. In this way, feedback is properly phased to produce continuous undamped oscillations.

Circuit operation. When the circuit is turned on, the capacitor is charged. When this capacitor is fully charged, it discharges through coils L1 and L2 setting up oscillations of frequency. The output voltage of the amplifier appears across L1 and feedbackvoltage across L2. The voltage across L2 is 180° out of phase with the voltage developed across L1 (Vout) as shown in Fig. IIt is easy to see that voltage fedback (i.e., voltage across L2) to the transistor provides positive feedback. A phase shift of 180° is produced by the transistor and a further phase shift of 180° is produced by L1 − L2 voltage divider. In this way, feedback is properly phased to produce continuous undamped

oscillations.

Wien Bridge Oscillator The Wien-bridge oscillator is the standard oscillator circuit for all frequencies in the range of 10 Hz to about 1 MHz. It is the most frequently used type of audio oscillator as the output is free from circuit fluctuations and ambient temperature. Fig. shows the circuit of Wien bridge oscillator. It is essentially a two-stage amplifier with R-C bridge circuit. The bridge circuit has the arms R1C1, R3, R2C2 and tungsten lamp Lp. Resistances R3 and Lp are used to stabilise the amplitude of the output. The transistor T1 serves as an oscillator and amplifier while the other transistor T2 serves as an inverter (i.e. to produce a phase shift of 180º). The circuit uses positive and negative feedbacks. The positive feedback is through R1C1, C2R2 to the transistor T1. The negative feedback is through the voltage divider to the input of transistor T2. The frequency of oscillations is determined by the series element R1C1 and parallel element R2C2 of the bridge.

When the circuit is started, bridge circuit produces oscillations of frequency. The two transistors produce a total phase shift of 360º so that proper positive feedback is ensured.

The negative feedback in the circuit ensures constant output. This is achieved by the temperature sensitive tungsten lamp Lp. Its resistance increases with current.

Should the amplitude of output tend to increase, more current would provide more negative feedback. The result is that the output would return to original value. A reverse action would take place if the output tends to decrease.

Advantages

(i) It gives constant output.(ii) The circuit works quite easily.(iii) The overall gain is high because of two transistors.(iv) The frequency of oscillations can be easily changed by using a potentiometer.

Disadvantages

(i) The circuit requires two transistors and a large number of components.(ii) It cannot generate very high frequencies.

• It consists of a transformer having primary and secondary circuits.

• The primary circuits consist of a battery which gives variable current, a npn transistor and a tank circuit consisting of inductance and capacitance connected in parallel.

• The secondary circuit consists of a coil L which is given to a Piezo-electric crystal.

Principle: Based on the principle of inverse piezo-electric

effect. When charges are given along the electrical axis ie, x-axis, then ultrasonic waves are produced along the mechanical axis ie, y-axis.

Working: • The value of C1 in the tank circuit adjusted so that

the frequency of tank circuit will become equal to the natural frequency of the vibrating crystal

• Due to this resonance occurs’• This produces ultrasonic waves.

Condition for resonance

Advantages• It can produce ultrasonic waves of frequency upto 500

MHz.• It is not affected by temperature and humidity.• It can produce high power ultrasonic waves.• It can produce longitudinal as well as transverse

ultrasonic waves.• It is more efficient that magnetostriction oscillator.

Didadvantages• The cost of piezo-electric crystal is high• Cutting and shaping the piezo-electric crystal is very

difficult.