EE 3111 Lab 7.1 BJT Amplifiers
BJT Amplifier Device/circuit that alters the amplitude of a signal, while keeping input waveform shape BJT amplifiers run the BJT in active mode. Forward current gain is determined by the transistor, e.g. BJT gain β=100 Forward voltage gain is determined by β and circuit elements Contrast this with Op-Amps which require feedback to completely determine the gain We will build four amplifiers
BJT Amplifier Can build common base, common collector or common emitter amplifiers We have two common emitter, one common collector and one common base here Different configurations have different gains and input/output impedance Input and output impedance can be very important depending on application!
BJT Model Active mode, low-frequency model R BE is input resistance g m is transconductance R CE is output resistance r be r ce
BJT Inverter Very basic amplifier Current is amplified by β, voltage amplified by function of RC, RE and BJT parameters, and is negative of input Can t control input impedance Common Emitter Amplifier Base is input, Collector is output, Emitter is common Current gain i ooo = β RC 4.7K Voltage gain vii = βr C rbb Input impedance v ii = r bb RB 50K Output impedance iooo = R C
BJT Inverter with Emitter Resistor Can increase input impedance at the cost of voltage gain Hi-Fi audio amplifiers have high input impedance High Frequency communications systems and Power Transmission systems need matching impedance of load, line and source Common Emitter Amplifier Current gain i ooo = β RC 4.7K Voltage gain vii = βr C r bb + β + 1 R E R C R E RB 50K Input impedance v ii = r bb + β + 1 R E βr E Output impedance iooo = R C
BJT Voltage Follower Very basic amplifier Current is amplified by β, voltage amplified by function of RC, RE and BJT parameters Gain is about 1, so it is used as a buffer and impedance matching Common Collector Amplifier Base is input, Emitter is output, Collector is common Current gain i ooo = β + 1 β RB 3.3K Voltage gain Input impedance vii = g mr E g m R E + 1 1 v ii = r bb + β + 1 R E β + 1 Output impedance iooo 1 g m + R B β
BJT Current Follower Very basic amplifier Current gain is about 1, voltage amplified by function of R C Large output impedance use as current buffer or current source Used as voltage amplifier high frequency applications Common Base Amplifier Emitter is input, Collector is output, Base is common RB 4.7K Current gain Voltage gain Input impedance i ooo = vii r bb + βr cc r bb + (β + 1)r cc 1 = (g mr cc + 1)R C R C + r cc v ii 1 g m g m R C Output impedance iooo R C r cc
BJT Parameter Procedure For IV curve of BJT and solving parameters, use diagram on the.vi screen, not the invertor diagram Calculate β f from IV curves ( 100) g m : Use power source for V CE, a source meter to provide I b, a source meter to measure I c. Use scope to measure V BE, (measure average voltage). Add ± 2µA to the Base current you found earlier and record the differences to calculate g m ( 0.15) r be : Use power source for V CE, a source meter to provide I b, a source meter to measure V be. Add ± 2µA to the Base current you found earlier and record the differences to calculate r be ( 5KΩ)
BJT Parameter Procedure β o : Use power source for V CC, a source meter to provide I b, a source meter to measure V be. Add ± 2µA to the Base current you found earlier and record differences to calculate β o ( 9) r ce : Use power source for V CC, a source meter to provide I b, a source meter to measure I C. Note that in this circuit V CE =V CC. Add ± 10mV to V CC and record differences to calculate β o ( 5KΩ) V SAT can be determined from IV curve, ( 0.3V)
Amplifier Procedures Note different resistor values from instruction pdf! Tranchar.vi uses two source meters and you need power source for Vcc For first three amplifiers, set start V=0, final V=5, step =0.2. Change the start or end to get a good view of the constant region, which is close to linear. Calculate the gain from the slope. For Current Follower, use start V=-5, final V=0, step =0.2. WHY? The DC offset for AC analysis should be roughly the middle V in on the linear region.
Amplifier Procedures Scope Vin and Vout and measure the V PP of both to recalculate the gain. Start with DC offset for Vin that you solved for. If you don t get a good gain (i.e. Vout is very small and/or noisy), raise or lower V in by 10% increments until V out stops getting bigger.