Discussion 6 Notes: Common Gate Amplifier (10/22/05)
1. Draw the DC biased Common Gate (CG) Amplifier as in Fig 8.38a, page 513
2. Discuss the skills of drawing small-signal model of the circuit as in Fig 8.39, page
514
• Meaning of constant voltage and current source in small signal circuit
(grounded and open circuit respectively. Why? No change of voltage and
current at those nodes!)
• Discuss the orders of magnitude of gm, gmb, ro , roc, RL, RS (gm = 10 gmb, gm
~ 1mS, ro ~1MΩ , ro << roc (because we can use cascade etc. for current
source), RL =>0 RS => inf for current amp and RL =>inf RS => 0, for
voltage amplifier)
• Discuss the meaning of gmb (change VSB => change VT h => change ID)
3. Draw the small signal circuit for deriving AV = vout / vin . Don’t have to derive.
Reinforce the concepts of drawing small signal circuits and also the definition of
AV. Remember to include series RS and parallel RL.
Give the answer: AV = (RL//roc//ro )(gm+gmb+1/ro )/(1+RS(gm+gmb+1/ro )
(RL//roc//ro )/(RL//roc)).
Simplification: Let Rs=>0, RL=> inf, also 1/ro << gm, roc >> ro
AV = roc (gm+gmb) (This simplified version has been derived in Lecture note)
4. Another way to find AV
• AV = vout /vin = (vout /iout ) (iout /iin ) (iin /vin ) (what are these? Rout AI 1/Rin )
• So, we have to find Rout, AI and Rin
• We know that the CG amplifier is a unity current gain amplifier if
everything is ideal. Therefore, we can model it as the 2-port network
shown in Fig. 8.42, page 517 (Draw the circuit, don’t give the exact value
of Rin and Rout yet)
• Apply a current source (with RS in parallel) and parallel RL as the output,
find AI = iout /iin .
• Apply current divider concept and get AI = RS/(Rin +RS) * Rout / (Rout + RL)
• Find Rin and Rout . Draw both Figures 8.41 and 8.40. Explain the
definitions and when RS, RL have to be attached.
• Both derived in lecture note already. But repeat either one of them to
reinforce the concept and show them the skill of derivation. (KCL, KVL)
• Rin = (1+(roc//RL)/ro )/(1/ro +gm+gmb) ~ 1/(gm+gmb) (which approximations
have we used?)
• Rout = roc// (R S (ro /RS + gmro + gmbro +1)) ~ roc // (ro (1+gmRs))
5. Finding AV:
• AV = Rout /Rin * RS/(Rin +RS) * Rout / (Rout + RL)
• Substitute Rout and Rin into AV
Use the assumption we used to derive AV (Let Rs=>0, RL=> inf, also 1/ro << gm,
roc >> ro ), AV = roc (gm + gmb)
6. The second method provides another path to calculate AV. This is especially
useful when we are only interested in approximated values.