POWER AMPLIFIERS ABOUT ANALOG ELECTRONICS, Summaries of Power Electronics

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Output Stages and Power

Amplifiers

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Output Stage

 Output Stage is the final stage of the amplifier.  Output resistance should be as low as possible.  Relatively large signals are present.  Small-signal models are not applicable.  Total Harmonic Distortion (THD) is a measure of goodness.  THD is the rms value of the harmonic components of the output signal as a percentage of the rms of the fundamental.  It is important to deliver the required power to the load efficiently.  Power dissipation on the output-stage transistors must be as low as possible. Asst.Prof.Dr.Özge ŞAHİN

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CLASSIFICATION OF OUTPUT STAGES

 Output stages are classified according to the collector current waveform.

Class A stage:

 Class A stage is biased at a current I (^) c greater than the amplitude of the signal current İc.

 The transistor in a class A stage conducts for the entire cycle of the input signal.

 The conduction angle is 360 °.

 All CE-CB-CC amplifiers studied earlier belong to Class A category. (^) Asst.Prof.Dr.Özge ŞAHİN

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Class B stage:

 The transistor in a class B stage is biased at zero DC current and it conducts for only half the cycle of the input signal.

 The conduction angle is 180 °.

 For the other half cycle, another transistor operates also in B class if necessary.

Asst.Prof.Dr.Özge ŞAHİN

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Class C stage:

 The conduction angle is less than 180 °.

 The output current is in pulsating

waveform.

 To obtain a sinusoidal output voltage, a

parallel LC circuit is necessary tuned to

the frequency of the input signal.

 Although BJT has been used for the

above mentioned classification, the

same classification can be used for the

output stages implemented with

MOSFETs.

Figure 14.2 An emitter follower ( Q 1 ) biased with a constant current I supplied by transistor Q 2. 8

CLASS A OUTPUT STAGE

Because of its low output resistance, Emitter follower(CC) is the most popular Class A output stage. Transfer Characteristic

 In Figure 14.2 an emitter follower is shown. It is biased with a constant current “I”.

 Since i E1=I+i L, I must be greater

than the largest negative load current in order to maintain class A operation.

 The transfer characteristic of the emitter follower is described by:

vO =vI-vBE.

Asst.Prof.Dr.Özge ŞAHİN

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 The positive limit is is determined by the saturation of Q 1 : v (^) Omax =VCC-V (^) CE1sat

 In negative direction the limit is determined ;

 either by Q 1 turning of: v (^) Omin=-IRL  or by Q 2 saturating: v (^) Omin= -VCC + VCE2sat

 The absolutely lowest output voltage is given as follows and it is achieved when “I” is greater than the load current.:

Asst.Prof.Dr.Özge ŞAHİN

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Signal Waveforms

 Neglecting V CEsat, if the bias current I is

properly selected the output voltage can

swing from –V CC to +V CC. (Fig. 14.4a)

 Fig. 14.4b shows the waveform of

vCE1=VCC – vO

Asst.Prof.Dr.Özge ŞAHİN

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 The quiescent power dissipation in Q 1 is equal to the maximum instantaneous ones.

 It is equal to VCCI and occurs at no input signal.

 The power dissipation in Q 1 depends on the value of R (^) L.

 Consider the extreme case when RL=∞.

 In this case, İc 1 =I and the power dissipation in Q 1 is VCCI.

 The most dangerous extreme case is short circuit condition RL=.

 In normal operation, the maximum power dissipation occurs when

v (^) O= - VCC

 In this case, v (^) CE1 = 2 VCC

 The maximum power dissipation in Q 1 is PD1 =2VCCI.

Power Dissipation:

Asst.Prof.Dr.Özge ŞAHİN

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 The power dissipation in Q 2 also must be taken into

account.

 Since Q 2 conducts a constant curret I , the maximum

value of vCE 2 = 2 V CC

 The maximum power dissipation in Q 2 is P D2=2VCC I.

Asst.Prof.Dr.Özge ŞAHİN

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CLASS B OUTPUT STAGE

 A class B output stage is shown in Fig. 14.5.

 It consists of a complementary pair of transistors (one NPN and one PNP). Circuit Operation:

 If v (^) I =0 both transistors are cut off.

 As v (^) I goes positive and exceeds V (^) BEγ≈0.5V QN conducts and supplies the load current. Q (^) P will be cut off.

 As v (^) I goes negative and exceeds V (^) EBγ≈0.5V Q (^) P conducts and supplies the load current in opposite direction. Q (^) N will be cut off.

 The transistors operate in a “push-pull” form. QN pushes current, and QPpulls current from the load.

Figure 14.5 A class B output stage.

Asst.Prof.Dr.Özge ŞAHİN

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Transfer Characteristic of the Class B

output Stage:

As v (^) I goes positive and exceeds V (^) BEγ ≈0.5V QN conducts and supplies the load current. QP will be cut off.

As vI goes negative and exceeds V (^) EBγ ≈0.5V QP conducts and supplies the load current in opposite direction. QN will be cut off.

Asst.Prof.Dr.Özge ŞAHİN

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 The average load power will be (crossover distortion is neglected):

• The peak of the load current is ṼO /RL.
• Thus, the average of this current will be ṼO /πRL.
• The average power drawn from each power supplies , the total supply power and the efficiency will be given by:

 The maximum efficiency is obtained near the saturation limit of the power transistors, when v (^) Omax=VCC-VCEsat≈VCC.

 At this peak value of the output voltage the efficiency is:

 The maximum average power available from the class B output stage is:

Power Conversion Efficiency (Class B)

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Power Dissipation:

 Unlike the class A stage, the quiescent power dissipation of the class B stage is zero, without any input signal.

 The average power dissipated in the class B is given by P (^) D=PS-PL.

 The half of PD is dissipated in Q (^) N and the other half in Q (^) P.

 Since P (^) D depends on ṼO we must find the worst case dissipation P (^) dmax.

 We can find this worst case value by differentiating of the above given equation with respect to ṼO and equating the derivative to zero.

 At the point of P (^) Dmax the efficiency is calculated as: η =50%