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BJT Amplifier

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					                 BJT Amplifier

•   Bipolar - electron and hole
•   Two types: npn and pnp
•   Current-controlled
•   Low input impedance
•   High voltage gain

http://www.mtmi.vu.lt/pfk/funkc_dariniai/transistor/bipola
   r_transistor.htm
http://www.ibiblio.org/obp/books/socratic/output/bjt2.pdf
                   Data Sheet
•   2N3904
•   npn
•   hFE = 180
•   VBE = 0.65 V
             Basic Characteristics

                          Active Region




Saturation Region
    Basic Characteristics

 A small base current controls a
  large collector current.
 Current amplification
  Curve Tracer Measurements
• IC versus VCE
            Curve Tracer Menu
 Device characteristics
 Menu
    -   Function: Acquisition
    -   Type: NPN
    -   Vce max: 20 V
    -   Ic max: 10 mA
    -   Ib/step: 10 mA
    -   Steps: 10
    -   Rload: 0.25 W
    -   Pmax: 0.1 W
   Screen print
             Basic Characteristics

                          Active Region




Saturation Region
  Curve Tracer Measurements
• IB versus VBE - Use the diode menu.
  Connect the base pin to collector socket.
  Leave the collector pin float.
IB versus VBE
• The base-emitter junction is forward
  biased, VBE = 0.6 - 0.7 V.
            Three Models
• 1. DC bias design - Q-point
• 2. h-parameter model - curve tracer
• 3. Hybrid-p model - ac analysis
          DC Bias Model




• Typical values:
  – Von = 0.65 V
  – RBB = 1 kW
  – dc = 150-180
     h-Parameter Model –
       Curve Tracer Plot




• Typical values:
  – hFE = 180
  – hIE = 1 kW, hOE > 40 kW
    Curve Tracer to h-Parameter
•   hIE = DVBE / DIB
•   hRE = DVBE / DVCE
•   hFE = DIC / DIB
•   hOE = DIC / DVCE
       Hybrid-p Model –
      AC Gain Calculation




• Typical values:
  – gm = 38.9·IC
  – rp = 1 kW, rx = 0, rC = > 40 kW
• Capacitors - High frequency roll off
                  Relations
•   hFE = ac = gmrp
•   hIE = rp + rx
•   hOE = 1 / rc
•   hRE = 0
BJT Common Emitter Amp.
           Circuit Characteristics
•   Av     =   gm RC
•   gm     =   38.9 IC
•   Zin    =   (rx + rp) || rB
•   Zout   =   RC || rc
         Effect of Capacitor
• Capacitors serve as open circuit for dc and
  short circuit for ac.
• There are coupling capacitors and the
  bypass capacitor which limit the low
  frequency response.
• There are junction capacitances which
  limit the high frequency response.
         Design Procedures
•   Design Goal              x200
•   Select VCC = 10-20 V.    15 V
•   Set VCE = VCC / 2.       7.5 V
•   Pick RE = 100-500 W.     220 W
•   RC = Av / gm             ~1.5 k W
•   IC (RC+RE) + VCE = VCC   4.5 mA
         Design Procedures
•   IB = IC / hFE      22 mA
•   VE = IC RE         1V
•   VB = VE + 0.65     1.65 V
•   Pick R2.           330 kW
•   Calculate R1.      ~500 kW
            Optimization
• Gain
  – Adjust power supply
• Maximum output swing without clipping
  – Adjust bias
• More gain?
  – Higher current, lower VCE
                       Load Lines
• DC Load line passes through the Q-point with a slope
  of - (RE+Rc)-1.
• AC Load line passes through the Q-point with a slope
  of - (Rc)-1.
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/loadline.html
              PSPICE
• Frequency response - bandwidth
• Transient response - waveform
  distortion, clipping
       Experimental Procedures

• Curve tracer measurements
    – Determine hFE.
    – Determine hOE.
    – Determine hIE (rp).
•   Calculate gm from hFE and rp.
•   Design circuit
•   Perform simulations
•   Build and characterize circuit
•   Optimization
Common Emitter Amplifier
Common Emitter Amplifier
             Measurements

• Frequency response
  – Scan frequency
  – Plot in a semi-log plot.
• Maximum swing without clipping
• Transient response
  – Drive with a square wave
• Output impedance
• Input impedance
    Resistor Transistor Logic




• Operate in the saturation regime
• Speed, delay, fan out, and power
  consumption are primary concerns.
  – Two resistors for optimization

  http://www.uic.edu/classes/ece/ece340/experiments/
    old.experiments/EECS340lab9.html

				
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