Electronics II (ELE 343 – Lecture 2)

1 Review of Common Base (CB) Amplifier

➢ AV, Rin, and Rout?

➢ In common base topology, where the base terminal is biased with a fixed voltage, emitter is fed with a signal, and collector is the output.

2 Review of Emitter Follower (CC Amplifier)

➢ AV, Rin, and Rout?

3 Review of Current Sources

▪ Ideal Current Sources

(Definition) A current source is an electronic circuit that delivers or absorbs an electric current which is independent of the voltage across it.

[I-V curve] I

I V

V

What is the output impedance for the current source? ➔ Ideally, it’s infinite. What happens if the output impedance is not infinite? 4 Example: Output Impedance

➢ We wish to increase the output resistance of the bipolar cascode of by a factor of two through the use of resistive degeneration in

the emitter of Q2. Determine the required value of the degeneration resistor if Q1 and Q2 are identical.

➢ Rout and RoutA?

➢ Typically, r is smaller than rO, so in general it is impossible to double the output impedance by degenerating Q2 with a resistor. 5 Example: Mixed BJT/MOS Cascode

2

1

➢ RoutA? ➢ RoutB?

6 PNP Cascode Stage

 Degeneration Tr

 Cascode Tr

➢ Rout?

7 Another Interpretation of Bipolar Cascode

➢ Instead of treating cascode as Q2 degenerating Q1, we can also think of it as Q1 stacking on top of Q2 (current source) to boost Q2’s output impedance.

8 False Cascodes

➢ Rout?

➢ When the emitter of Q1 is connected to the emitter of Q2, it’s no longer a cascode since Q2 becomes a diode-connected device instead of a current source. 9 MOS Cascode Stage

➢ Rout?

10 Example: MOS Cascades

➢ Design an NMOS cascode for an output impedance of 500

kΩ and a current of 0.5 mA. For simplicity, assume M1 and M2 are identical (they need not be). 2 Assume μnCox = 100 μA/V and λ = 0.1V−1.

11 Another Interpretation of MOS Cascode

➢ Similar to its bipolar counterpart, MOS cascode can be thought of as stacking a transistor on top of a current source. ➢ Unlike bipolar cascode, the output impedance is not limited by .

12 Example: Parasitic Resistance

➢ During manufacturing, a large parasitic resistor RP has appeared in a cascode as shown in the figure. Determine the output resistance.

➢ Rout?

➢ RP will lower the output impedance, since its parallel combination with rO1 will always be lower than rO1.

13 Electronics II (ELE 343 – Lecture 3)

14 Short-Circuit Transconductance

➢ The short-circuit transconductance of a circuit measures its strength in converting input voltage to output current.

15 Transconductance Example

16 Derivation of Voltage Gain

➢ By representing a linear circuit with its Norton equivalent,

the relationship between Vout and Vin can be expressed by the product of Gm and Rout.

17 Example: Voltage Gain of CE Stage

18 Comparison between Bipolar Cascode and CE Stage

➢ Since the output impedance of bipolar cascode is higher than that of the CE stage, we would expect its voltage gain to be higher as well.

19 Voltage Gain of Bipolar Cascode Amplifier

➢ Since rO is much larger than 1/gm, most of IC,Q1 flows into the diode-connected Q2. Using Rout as before, AV is easily calculated.

20 Example: Bipolar cascode

➢ The bipolar cascode is biased at a current of 1 mA. If VA = 5 V and β = 100 for both transistors, determine the voltage gain. Assume the load is an ideal current source.

21 Alternate View of Cascode Amplifier

➢ A bipolar cascode amplifier is also a CE stage in series with a CB stage.

22 Practical Cascode Stage

➢ Since no current source can be ideal, the output impedance drops.

23 Improved Cascode Stage

➢ Rout?

➢ In order to preserve the high output impedance, a cascode PNP current source is used.

24 Example: Bipolar cascode

➢ Suppose the circuit incorporates a cascode load using pnp transistors with VA = 4 V and β = 50. What is the voltage gain? (A collector current of approximately 1 mA, npn transistors

with VA = 5 V and β = 100.)

25 MOS Cascode Amplifier

26 Improved MOS Cascode Amplifier

➢ Rout, RoN, and RoP?

➢ Similar to its bipolar counterpart, the output impedance of a MOS cascode amplifier can be improved by using a PMOS cascode current source.

27 Example: MOSFET cascode

➢ The MOSFET cascode amplifier incorporates the following

device parameters: (W/L)1,2= 30, (W/L)3,4= 40, ID1 = ··· = ID4 = 2 2 0.5 mA. If μnCox = 100 μA/V , μpCox = 50 μA/V , λn = 0.1V−1 and λp = 0.15 V−1, determine the voltage gain.

28