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

• 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 Amplifier

• Active mode, low-frequency model • RBE is input resistance • gm is transconductance RCE is output resistance

BJT Model

rbe rce

• 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

BJT Inverter

Current gain 𝑖𝑜𝑜𝑜

𝑖𝑖𝑖 = 𝛽

Voltage gain 𝑣𝑜𝑜𝑜 𝑣𝑖𝑖 = −𝛽𝑅𝐶 𝑟𝑏𝑏

Input impedance

𝑣𝑖𝑖𝑖𝑖𝑖 = 𝑟𝑏𝑏

Output impedance

𝑣𝑜𝑜𝑜𝑖𝑜𝑜𝑜 = 𝑅𝐶

RC 4.7K

RB 50K

• 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

BJT Inverter with Emitter Resistor

Current gain 𝑖𝑜𝑜𝑜

𝑖𝑖𝑖 = 𝛽

Voltage gain 𝑣𝑜𝑜𝑜 𝑣𝑖𝑖 = −

𝛽𝑅𝐶𝑟𝑏𝑏 + 𝛽 + 1 𝑅𝐸

≈ −𝑅𝐶𝑅𝐸

Input impedance 𝑣𝑖𝑖

𝑖𝑖𝑖 = 𝑟𝑏𝑏 + 𝛽 + 1 𝑅𝐸 ≈ 𝛽𝑅𝐸

Output impedance 𝑣𝑜𝑜𝑜

𝑖𝑜𝑜𝑜 = 𝑅𝐶

RC 4.7K

RB 50K

BJT Voltage Follower

Current gain 𝑖𝑜𝑜𝑜

𝑖𝑖𝑖 = 𝛽 + 1 ≈ 𝛽

Voltage gain 𝑣𝑜𝑜𝑜 𝑣𝑖𝑖 =

𝑔𝑚𝑅𝐸𝑔𝑚𝑅𝐸 + 1 ≈ 1

Input impedance 𝑣𝑖𝑖

𝑖𝑖𝑖 = 𝑟𝑏𝑏 + 𝛽 + 1 𝑅𝐸≈ 𝛽 + 1

Output impedance 𝑣𝑜𝑜𝑜

𝑖𝑜𝑜𝑜 ≈ 1 𝑔𝑚 + 𝑅𝐵𝛽

• 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

RB 3.3K

BJT Current Follower

Current gain 𝑖𝑜𝑜𝑜

𝑖𝑖𝑖 =𝑟𝑏𝑏 + 𝛽𝑟𝑐𝑏

𝑟𝑏𝑏 + (𝛽 + 1)𝑟𝑐𝑏≈ 1

Voltage gain 𝑣𝑜𝑜𝑜 𝑣𝑖𝑖 = −

(𝑔𝑚𝑟𝑐𝑏 + 1)𝑅𝐶𝑅𝐶 + 𝑟𝑐𝑏

≈ 𝑔𝑚𝑅𝐶

Input impedance 𝑣𝑖𝑖

𝑖𝑖𝑖 ≈1𝑔𝑚

Output impedance 𝑣𝑜𝑜𝑜

𝑖𝑜𝑜𝑜 ≈ 𝑅𝐶||𝑟𝑐𝑏

• 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

• 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)

BJT Parameter Procedure

• 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.

Amplifier Procedures