240ln12 - OUTPUT STAGES OF AMPLIFIERS LN 12 Amplifiers...

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1 OUTPUT STAGES OF AMPLIFIERS LN 12 Amplifiers require output stages to provide sufficient power to interface with the external world, i.e. to transmit into space, or to drive a mechanical object such as a speaker, or a display device or another larger amplifier. The objectives in good output stage design are 1.ideal voltage source which means to minimize Ro 2.low distortion ( replicates the input with high fidelity and gain in voltage and current) 3.high efficiency which implies that the design will consume low quiescent power 4.high frequency operation ( it is difficult to have high frequency operation with high output power) V i I R Q1 Q2 Q3 R 3 R 2 R L V o + - V CC I Q R 1 V i = V be1 + V o V be1 = V T ln(I c1 /I s ) assuming R L < r o and Q1 in forward active I c1 = I Q + V o /R L if Q2 in forward active and β is large Then o s L o Q T i V I R V I V V + + = ln Emitter Follower Output Stage CLASS A -V cc
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2 -V cc + V CE2(sat) -V cc +V CE1(sat) +V be1 Q1 cutoff V be1 TRANSFER CHARACTERISTIC for CLASS A Output V cc – V CE(sat) Q1 saturated Q2 saturated V i V o -I Q R L2 R L2 R L1 V 1 <I Q R L2 V 2 >I Q R L2 V i t V 2 -V 2 -V 1 V 2 V 1 V 1 -V 1 Waveform clipping t DC Design V BE3 = V BE2 Q2 and Q3 have the same collector I c due to the current mirror I R = I Q = (V cc -V BE3 )/R 3 for R 1 =R 2 Note that for the situation I Q R L < -V CC + V CE2(sat) , Distortion (clipping) occurs because during the –ve voltage swing , Q1 will cut-off Note , for R 2 <R 1 , I R <I Q V o These are the input and output voltage waveforms to the Class A stage. Note that this is an emitter follower, so that the voltages essentially track each other (voltage gain very slightly less than 1) until the waveform clipping occurs as seen in the output waveform.
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3 OUTPUT CHARACTERISTIC & LOAD LINES FOR Q1 (Note BV ceo > 2V cc V cc I c1 V ce1 Q R L =R L2 R L =R L3 Vcc+I Q R L2 2V cc -V CE2(sat) R L =R L1 A B C Output curves for constant V be1 values 1 _ _ ˆ L L om R R for I = Maximum possible value of Vce1 ) Load line equation V ce1 = V cc - (I c1 – I Q ) R L Definitions : ˆˆ _ _ _ _ _ _ _ _ max_ _ _ _ _ _ _ _ _ max_ o om o om V zero to peak output voltage amplitude V is value I zero to
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This note was uploaded on 03/29/2011 for the course ECE 4201 taught by Professor N/a during the Spring '11 term at UConn.

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240ln12 - OUTPUT STAGES OF AMPLIFIERS LN 12 Amplifiers...

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