RECALL%20LECTURE%209

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RECALL LECTURE 9

• Introduction to BJT
• 3 modes of operation
• Cut-off
• Active
• Saturation
• Active mode operation of NPN

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NPN
PNP
IE IS e VBE / VT
IE IS e VEB / VT
IC ? IB
IC ? IE
IE IB( ? 1)
? ? / 1 - ?
? ? / ? 1
Based on KCL IE IC IB
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DC Analysis of BJT Circuit
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• DC analysis of BJT
• BE Loop (EB Loop) VBE for npn and VEB for pnp
• CE Loop (EC Loop) - VCE for npn and VEC for pnp
• When node voltages are known, branch current
  equations can be used.

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Common-Emitter Circuit

• The figures below is showing a common-emitter
  circuit with an npn transistor and the dc
  equivalent circuit.

• Assume that the B-E junction is forward biased,
  so the voltage drop across that junction is the
  cut-in or turn-on voltage VBE (on).

Unless given , always assume VBE 0.7V
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Common-Emitter Circuit

• The base current KVL at B-E loop

• Implicitly assuming that VBB gt VBE (on), which
  means that IB gt 0. When VBB lt VBE (on), the
  transistor is cut off and IB 0.

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Common-Emitter Circuit

• In the C-E loop of the circuit, we can use
  and

• Implicitly assuming that the transistor is biased
  in the forward-active mode.

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Examples

• BJT DC Analysis

9
Common-Emitter Circuit
Example Calculate the base, collector and
emitter currents and the C-E voltage for a
common-emitter circuit by considering VBB 4 V,
RB 220kO, RC 2 kO, VCC 10 V, VBE (on) 0.7
V and ß 200.
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BJT Circuits at DC
? 0.99
KVL at BE loop 0.7 IERE 4 0 IE 3.3 /
3.3 1 mA Hence, IC ? IE 0.99 mA IB
IE IC 0.01 mA
KVL at CE loop ICRC VCE IERE 10 0
VCE 10 3.3 4.653 2.047 V
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Common-Emitter Circuit - PNP
Example Find IB, IC, IE and RC such that VEC
½ VCC for a common-emitter circuit . Consider
VBB 1.5 V, RB 580 O, VCC 5 V, VEB (on)
0.6 V, ß 100.
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EXAMPLE
Given ? 75 and VEC 6V. Find the values of the
labelled parameters, RC and IE,