Lecture 9 Power Amplifiers -Class B

Transcription

1 1 Lecture 9 Power Amplifiers -Class B

2 Class-B Amplifier :- v in A v v out Class-B operation :- Common-collector class-b amplifier :- +V +0.7V Transistor conducts v in v out Transistor off 0 Class-B amplifier provides an output signal varying over one-half the input signal cycle + zero phase shift.?? where is the Q-point on the load line???

3 3 The dc bias point for class-b amplifier is therefore at 0 volt. i.e. biased at cutoff :- = 0 and CQ V = V CEQ CE(off ) The advantage of a class-b amplifier is that the collector current is zero when the input signal to the amplifier is zero. Therefore the transistor dissipate no power in the quiescent condition, i.e. more efficient!! class-b amplifier was developed to improve on the low efficiency rating of the class-a amplifier. Obviously, the output is not a faithful reproduction of the input if only one half-cycle is present. Therefore, a two-transistor configuration, is necessary to get a sufficiently good reproduction of the input waveform. This amplifier configuration is known as push-pull emitter follower (pushpull amplifier) or complementary-symmetry amplifier.

4 Push-Pull Operation :- 4 Class-B push-pull amplifier circuit :- +V = on R L Q = on v in Q -V The circuit configuration feature is the use of complementary transistors, i.e. one of the transistors is a npn and the other is a pnp. The term push-pull comes from the fact that two transistors in a class-b amplifier conduct on alternating half-cycles of the input. The combined half-cycles then provide an output for a full of operation. Note :- Need dual-polarity power supplies.

5 5 No nput :- When the transistor is in its quiescent state (no input), both transistors are biased at cutoff. Positive nput :- During the positive half-cycle of the input signal, is biased above cutoff, and conduction results through the transistor R L. During this time, Q is still biased at cutoff. provide output on the positive-output half-cycle. Negative nput :- During the negative half-cycle of the input signal, is returned to the cutoff state, and Q is biased above cutoff. As a result, conduction of Q start to built while remains off. provide output on the negative-output half-cycle. The combined half-cycles then provide an output for a full of operation. t is important that the two transistors in a push-pull configuration be carefully matched.?? WHY??

6 Crossover Distortion :- 6 Among the disadvantages of a class-b amplifier is that the nonlinear cutoff region is included in the operation range. Because of the biasing arrangement, class-b amplifiers are subject to a type of distortion. When V B = 0, the input signal voltage must exceed V BE before a transistor conduct. Therefore, there is a time interval between the positive and negative alternations when neither transistor is conduction. The resulting distortion in the output waveform is quite common and is called crossover distortion. To prevent crossover distortion, both transistors will normally be biased at a level that is slightly above cutoff. Biasing both transistors slightly above cut-off will allow the amplifier to provide a linear output that contains no distortion. Class-AB Amplifier

7 Class-AB Amplifier :- 7 To eliminate crossover distortion, both transistors in the push-pull arrangement must be biased slightly above cut-off when there is no signal. This can be done with, for example, a voltage-divider arrangement. This variation of the class B push-pull amplifier is designated as class-ab. (1) Voltage-Divider Bias :- +V R 1 R = on R 3 R L Q v in Q = on R 4 Voltage-divider bias class-ab amplifier. Note R L is capacitively coupled (dual-polarity power supplies single-polarity power supply)

8 8 However, difficult to maintain a stable bias point with this circuit due to changes in V BE over temperature changes. (i.e. Δtemp ΔQ-point ) A more stable arrangement () Diode Biasing Circuit :- +V R 1 D 1 D R L Q R When the diode characteristics of D 1 and D are closely matched to the transconductance characteristics of the transistors, a stable bias can be maintained over temperature. This can be also be accomplished by using the base-emitter junction of two additional transistors instead of D 1 and D. Although technically incorrect, class-ab amplifiers are often referred to as class-b in common practice.

9 DC Operating Characteristics :- 9 +V + R 1 V + D D V Q V R - - DC equivalent circuit. Assume :- (i) R = R, 1 (ii) transconductance characteristic of the diodes and the transistors are identical. Q-point :- V BE1Q V BEQ, V CE1Q V CEQ, C1Q CQ, V V CEQ Because both transistors are biased near cutoff :- = 0 CQ

10 AC Operating Characteristics :- 10 C (ma) C(sat) ac load line i c V CEQ = V Q R L R 1 R v ce Q = on V CE = on 0V V CEQ V AC equivalent circuit. AC load line. Under maximum conditions :- both transistors and Q are alternately driven from near cutoff to near saturation Q 1 V V Q 0 V and :- v ce ( peak ) V CEQ V V CEQ v ce( peak ) CEQ = = = C ( sat ) R L V R L V R L

11 n ac operation V v o R 1 V v L 0 0 D 1 = on D C C R L Q Q = on v in R v in = +ve When input v in is positive and is conducting, current is drawn from the power supply and flows through to the load. v in = -ve When is cut-off by a negative input, no current can flow from the supply. At those times, Q is conducting and capacitor C C discharges through that transistor. Thus, current flows from the load, through C C, and through Q to ground whenever the input is negative. The R L C C time constant must be much great than the period of the lowest signal frequency. The lower cut-off frequency due to C C is given by f 1 = πr L C C

12 Power Calculations :- 1 (1) DC nput Power :- The total (dc) input power comes from the source :- V P ( dc) = V i = + C ( ave ) 1 >> ) C ave C ( ave ) P = ( dc) V i C ( ave ) ( ( ) 1 The total current drawn from the supply is the sum of the average collector current and the current through the amplifier base circuit. The average value of the current through the collector of is given as - c c(sat) c(ave) T/ T t 1 T C ( sat ) = dt C ( ave) = 0 C T π i.e. just a standard ave equation for the half-wave rectifier. P ( dc) i = V C ( sat ) π V P ( dc) = i π C ( sat )

13 13 () Maximum AC Output Power :- C (ma) C(sat) ac load line i c V CEQ = V Q R L R 1 R v ce Q = on V CE = on 0V V CEQ V The class-b amplifier has the same (ac) output power characteristics as the class-a amplifier :- vo( rms ) P ( ac) = i v = o c ( rms ) o( rms ) RL The maximum load power is: P = ( ac) max i v o c (max)( rms ) o(max)( rms ) P ( ac) = o max P ( ac) = o max C ( sat ) C ( sat ) V V CEQ P ( ac) = o max C ( sat ) 4 V η = P o( ac ) P i ( dc ) 100% = V C ( sat ) 4 π V C ( sat ) 100% π = 100% = 79% 4 η max 79%

14 14 Power Amplifiers -Class C

15 Class-C Amplifier :- 15 V in A v V out Class-C amplifier operation (inverting). Basic Operation :- The transistor is biased with a negative V BE. Thus it will conduct only when the input signal is above a specified positive value. i.e. transistor ON when V in > V BB + V BE

16 16 A class-c amplifier load line, where V BEQ is set to a negative value. The power dissipation of the transistor in a class-c amplifier is low because it is on for only a small percentage of the input cycle. For example :- f a sinusoid forms the input to a class-c amplifier, the output consists of blips at the frequency of the input. Since this is a periodic signal, it contains a fundamental frequency component plus higher-frequency harmonics. f this signal is passed through an inductor-capacitor (LC) circuits tuned to be resonant at the fundamental frequency, the output is approximately a sinusoidal signal at the same frequency as the input. This approach is often used if the signal to be amplified is either a pure sinusoid or a more general signal with a limited range of frequencies. Class-C amplifiers are capable of providing large amounts of power, They are often used for transmitter power stages, such as radio or communications, where a tuned circuit is included to eliminate the higher harmonics in the output signal. η max > 98%

17 17 Tuned Class-C Amplifier :- Because the collector voltage (output) is not a replica of the inputs, the resistively loaded class-c amplifier is of no value in linear applications. t is therefore necessary to use a class-c amplifier with a parallel resonant circuit, as shown in Figure (a). The resonant frequency of the tuned circuit is determined by the formula :- f = 1 π LC The tuned circuit in the output will provide a full cycle of output signal for the fundamental or resonant frequency of the tuned circuit of the output. This type of operation is therefore limited to use at one fixed frequency, as occurs in a communications circuit, for example. Operation of a class-c circuit is not intended primarily for large-signal or power amplifiers.

18 18 Power Transistor Heat Sinking :- Power transistor can dissipate many watts. For example :- N3055, an inexpensive power transistor of great popularity, can dissipate as much as 115 watts if properly mounted. All power devices are packaged in cases that permit contact between a metal surface and an external heat sink. n most cases that metal surface of device is electrically connected to one terminal (e.g. for power transistors the case is always connected to the collector). nsulator :- insulated from heat sink, as is usually necessary, especially if several transistors are mounted on the same sink. Chassis or heat sink :- provides additional surface area to conduct heat away from the transistors more quickly to prevent overheating.

19 19 The whole point of heat sinking is to keep the transistor junction (or the junction of some other device) below some maximum specified operating temperature. For silicon transistors in metal packages the maximum junction temperature is usually 00 0 C, whereas for transistors in plastic packages it is usually C.

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21 1 Summary:- Power Amplifiers Amplifier Efficiency Amplifier Classification Class-A Amplifier Basic Operation Principle DC Operating Characteristics AC Operating Characteristics AC Load Line Amplifier Compliance Power Calculations Maximum Efficiency Class-B Amplifier Basic Operation Principle Push-Pull Emitter Follower Crossover Distortion Class-AB Amplifier Voltage-Divider Configuration Diode Bias Configuration DC Operating Characteristics AC Operating Characteristics Power Calculations Maximum Efficiency Class-C Amplifier Basic Operation Principle Tuned Class-C Amplifier Basic Operation Principle Power Transistor Heat Sinking