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Radio Frequency Amplifiers

VIEWS: 10 PAGES: 19

									Radio Frequency Amplifiers
    In this section of the course:
   Why do common emitter amplifiers often
    have a disappointingly low upper cut-off
    frequency ?
   Where are the hidden capacitances which
    cause the effect ?
   How can we design high frequency
    amplifiers ?
Frequency Response of Amplifiers
                       IB  0
                      VB  0 V
                      VE  0.5 V
                       IC  I E
                               0.5   15
                          
                                   RE
                           0.8 mA
                      VC  15  I C RC
                           8.6 V
   Voltage gain
        IC
 g m
        VT
       0 .8
     
       25
      32 mS
vout
      g m  RC || RL 
vin
      32  4.4
      140 (43 dB)
Input impedance

      1
 re      31 
      gm
   350
 rin  100 || 100 || re
     100 || 100 || 10
     8 k
Lower Cut-Off Frequency
                           vin
               i
                            1
                  RS             rin
                         j 2fC
               Cut - off when :
                             1
               RS  rin 
                           2fC
                               1
                f                       19 kHz
                        2  RS  rin C
NB. At 19 kHz:
        1
XC2 
      2fC2
                  1
       
         2 19000 100 10 9
        84 
           1
X C3   
         2fC3
                 1
       
         2 19000 10 10 6
        0.8 
Frequency Response
High Frequency Transistor Model
Values of Internal Capacitances
   Both capacitances vary depending on the bias
    conditions of the junctions.
   CBC is particularly variable due to wide range
    of possible VCE values.
   For simplicity, we will assume both
    capacitances are fixed – usually at a few
    picofarads.
Small Signal Transistor Models
   A popular technique for analysing transistor
    amplifiers involves small signal transistor
    models.
   A small signal (a.c.) equivalent circuit of the
    transistor is placed in the circuit.
   Circuit analysis now only involves primitive
    circuit elements.
 Simplified Hybrid- Model

 Simplest possible small signal transistor model.
 Suitable for low frequency analysis only.



                                         
                            r  re 
                                         gm
                                 IC
                            gm 
                                 VT
Using the Hybrid- Model
                           I C  0.8 mA
                                IC
                           g m
                                VT
                               0.8
                             
                               25
                              32 mS
                                    350
                           r     
                                g m 0.032
                              10 k
vout vout  g m v   4400
                           140
vin   v           v
High Frequency Hybrid- Model
Resistance, rbb
   rbb is the base spreading resistance.
   It is a real physical resistance.
   It represents the imperfect electrical contact
    between the base material and the terminal.
   Typical value < 100 .
Resistance, r
   r is the same small signal base-emitter
    resistance used in the simplified model.
   It is not a real resistance, but the small signal
    relationship between vbe and ib.

                                     VT
                r  re         
                             gm       IC
Resistance, ro
   In the simplified model, the collector appears
    to be an ideal current source.
   Of course, it isn’t ideal; a resistance, ro,
    appears in parallel.
   Usually, ro is much larger than the collector
    resistor and can be ignored.
Capacitances
   The two capacitances CBC and CBE correspond
    to the junction capacitances.
   Both vary with d.c. bias conditions.
   For simplicity, we’ll assume they are constant.
Summary
   Frequency response of amplifiers depends chiefly on
    the capacitances within the circuit.
   This includes internal capacitances of the transistor.
   Simplified hybrid- transistor model can help analyse
    a circuit.
   High frequency hybrid- model is a more accurate
    approximation.
   Next time : applying the high frequency hybrid-
    model.

								
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