ENGR337 Lab 2018 Spring

Lab11 CMOS and BJT Amplifiers

Common-source amplifiers using CMOS transistors and common-emmitter amplifiers using BJTs can be used as simple amplifiers in the front-end. They don't have the out-standing common-mode rejection ratio as the Op Amps, but they can still be found in many consumer eletronics where the noise rejection requirements are not that strict.

Some comments about CMOS and BJTs:
1) MOSFET is a 'square law' device, but BJT is not.
2) MOSFET is a 'voltage controlled current source', BJT is a 'current controlled curren source'.
3) The input impedance of MOSFET is higher than BJTs, and has less 'loading' effects.
4) Because of the input capacitance of MOSFETs, the bandwidth of MOSFET is lower than BJTs. So BJTs are used for radio-frequency devices.

Now, let's build common-emitter amplifiers using BJTs first:

We will use the NPN transistor 2N3903 for this experiment.

1. Build the circuit in LTSpice and simulate the input/output using '.tran' analysis (no calculations so far!). ** The 4.7u capacitors are used to remove the DC offset voltage and only passes AC signals.



Use the '.op' analysis to test the DC operating point that you need to get 'gm' (use your mouse to probe the interested nodes, use VBE=0.7V, VT=25mV).

Then use (gm * vbe) to calculate 'ic' and then vo1 (use the small signal model to calculate vbe). (** the following figure is just a model, don't build it in LTSpice)



Use the parameters you tested in the circuit to calculate the AC voltage gain and compare to the gain you got from the '.tran' analysis below in your simulation (yes, the gain is small):



The derivation of the AC gain can be found here.

2. Build the circuit on your bread board and use the similar method to calculate the AC gain of your amplifier.

3. To boost the gain, use a cascoding BJT amplifier structure like the following figure. Build it in LTSpice and on a bread board to get a higher amplification.





This is the end of this lab.