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
• RBE is input resistance
• gm is transconductance RCE is output resistance
rbe
rce
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
RC
4.7K
Voltage gain
Input
impedance
RB
50K
Output
impedance
𝑖𝑜𝑜𝑜
�𝑖 = 𝛽
𝑖𝑖
𝑣𝑜𝑜𝑜
𝛽𝑅
�𝑣𝑖𝑖 = − 𝐶�𝑟𝑏𝑏
𝑣𝑖𝑖
�𝑖 = 𝑟𝑏𝑏
𝑖𝑖
𝑣𝑜𝑜𝑜
= 𝑅𝐶
�𝑖
𝑜𝑜𝑜
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
RC
4.7K
RB
50K
Voltage gain
Input impedance
Output impedance
𝑖𝑜𝑜𝑜
�𝑖 = 𝛽
𝑖𝑖
𝛽𝑅𝐶
𝑅𝐶
≈−
+
𝛽
+
1
𝑅
𝑅𝐸
𝑏𝑏
𝐸
𝑣𝑜𝑜𝑜
�𝑣𝑖𝑖 = −
𝑟
𝑣𝑖𝑖
�𝑖 = 𝑟𝑏𝑏 + 𝛽 + 1 𝑅𝐸 ≈ 𝛽𝑅𝐸
𝑖𝑖
𝑣𝑜𝑜𝑜
= 𝑅𝐶
�𝑖
𝑜𝑜𝑜
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
RB
3.3K
Voltage gain
Input impedance
Output impedance
𝑖𝑜𝑜𝑜
�𝑖 = 𝛽 + 1 ≈ 𝛽
𝑖𝑖
𝑔𝑚 𝑅𝐸
𝑣𝑜𝑜𝑜
�𝑣𝑖𝑖 =
≈1
𝑔𝑚 𝑅𝐸 + 1
𝑣𝑖𝑖
�𝑖 = 𝑟𝑏𝑏 + 𝛽 + 1 𝑅𝐸
𝑖𝑖
≈𝛽+1
𝑣𝑜𝑜𝑜
𝑅
≈ 1�𝑔𝑚 + 𝐵�𝛽
�𝑖
𝑜𝑜𝑜
BJT Current Follower
• Very basic amplifier – Current gain is about 1, voltage amplified by
function of RC
• 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
Output impedance
𝑟𝑏𝑏 + 𝛽𝑟𝑐𝑐
𝑖𝑜𝑜𝑜
�𝑖 =
≈1
𝑖𝑖
𝑟𝑏𝑏 + (𝛽 + 1)𝑟𝑐𝑐
(𝑔𝑚 𝑟𝑐𝑐 + 1)𝑅𝐶
𝑣𝑜𝑜𝑜
�𝑣𝑖𝑖 = −
≈ 𝑔𝑚 𝑅𝐶
𝑅 +𝑟
𝐶
𝑐𝑐
1
𝑣𝑖𝑖
�𝑖 ≈
𝑖𝑖
𝑔𝑚
𝑣𝑜𝑜𝑜
≈ 𝑅𝐶 ||𝑟𝑐𝑐
�𝑖
𝑜𝑜𝑜
BJT Parameter Procedure
• For IV curve of BJT and solving parameters, use
diagram on the .vi screen, not the invertor diagram
• Calculate βf from IV curves (≈ 100)
• gm: Use power source for VCE, a source meter to
provide Ib, a source meter to measure Ic. Use scope
to measure VBE, (measure average voltage). Add ±
2µA to the Base current you found earlier and
record the differences to calculate gm (≈ 0.15)
• rbe: Use power source for VCE, a source meter to
provide Ib, a source meter to measure Vbe. Add ±
2µA to the Base current you found earlier and
record the differences to calculate rbe (≈ 5KΩ)
BJT Parameter Procedure
• βo : Use power source for VCC, a source meter to
provide Ib, a source meter to measure Vbe. Add ±
2µA to the Base current you found earlier and
record differences to calculate βo (≈ 9)
• rce: Use power source for VCC, a source meter to
provide Ib, a source meter to measure IC. Note that
in this circuit VCE=VCC . Add ± 10mV to VCC and record
differences to calculate βo (≈ 5KΩ)
• VSAT 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 Vin on the linear region.
Amplifier Procedures
• Scope Vin and Vout and measure the VPP 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 Vin by 10% increments
until Vout stops getting bigger.