ch. 6 electronics

• AC quantities


  Indicated with an italic subscript; rms values are assumed unless otherwise stated
• Resistance


  Identified with a lower case subscript when analyzed from an ac standpoint
• Linear amplifier


  Produces a replica of the input signal at the output
• The voltage waveform is inverted between the input and output but has the same shape
• AC load line


  Different than the dc load line because a capacitor looks open to dc but effectively acts as a short to ac. Thus the collector resistor appears to be in parallel with the load resistor.
• 1. parameters


  Can be used for detailed analysis of a BJT circuit. The simplified r-parameters give good results for most analysis work.
• re'


  An important r-parameter that appears as a small ac resistance between the base and emitter
• Common-emitter (CE) amplifier


  The input signal is applied to the base and the inverted output is taken from the collector. The emitter is common to ac signals.
• What is re' for the CE amplifier? Assume stiff voltage-divider bias.
• Greater gain stability can be achieved by adding a swamping resistor to the emitter circuit of the CE amplifier
• Multisim is a good way to check calculations
• Common-collector amplifier (emitter-follower)


  Has a voltage gain of approximately 1, but can have high input resistance and current gain. The input is applied to the base and taken from the emitter.
• A "rock solid" stiff voltage-divider is not always the best design
• Darlington pair


  Two transistors connected where the two transistors act as one "super β" transistor. Darlington transistors are available in a single package. Notice there are two diode drops from base to emitter.
• Sziklai pair


  A pnp and npn transistor connected, which has the advantage of only one diode drop between base and emitter.
• b = 200


  Transistor beta
• The problem is the power gain is less than 1!
• Darlington Pair


  Two transistors connected as shown, acting as one "super b" transistor
• Darlington Pair operation


  1. Two transistors connected

  2. Act as one "super b" transistor
• Darlington transistors are available in a single package
• There are two diode drops from base to emitter in a Darlington Pair
• Sziklai Pair


  A pnp and npn transistor connected, with only one diode drop between base and emitter
• IE2


  Approximately equal to βDC1βDC2IB1
• Common-Base (CB) Amplifier


  Used in applications where a low input impedance is acceptable, does not invert the signal
• C2


  Forces the base to be at ac ground
• Multistage Amplifiers


  Stages cascaded where the output of one drives another, input and output signals capacitively coupled
• Example two-stage direct-coupled amplifier circuit
• Differential Amplifier


  Has two inputs, amplifies the difference in the two input voltages
• Differential-mode inputs illustrated
• Diff-amps tend to reject common-mode signals, which are usually due to noise
• Ideally, the outputs are zero with common-mode inputs
• 1. parameter


  One of a set of BJT characteristic parameters
• Common-Emitter


  A BJT configuration in which the emitter is the common terminal to an ac signal
• ac ground


  A point in a circuit that appears as a ground to ac signals only
• Input Resistance


  The resistance seen by an ac source connected to the amplifier input
• Output Resistance


  The ac resistance looking in at the amplifier output
• Common-Collector


  A BJT configuration in which the emitter is the common terminal to an ac signal
• Differential Amplifier


  An amplifier in which the output is a function of the difference between two input voltages
• Common-Mode


  A condition where two signals applied to differential inputs are of the same phase, frequency and amplitude
• The equation for finding the ac emitter resistance of a BJT is re' = 25 mV/IE