Cascode Stage

OUTLINE Review of BJT Amplifiers Cascode Stage Reading: Chapter 9.1

Review: BJT Amplifier Design A BJT amplifier circuit should be designed to 1. ensure that the BJT operates in the active mode, 2. allow the desired level of DC current to flow, and 3. couple to a small-signal input source and to an output “load”.  Proper “DC biasing” is required! (DC analysis using large-signal BJT model) Key amplifier parameters: (AC analysis using small-signal BJT model) Voltage gain A v  v out /v in Input resistance R in  resistance seen between the input node and ground (with output terminal floating) Output resistance R out  resistance seen between the output node and ground (with input terminal grounded)

Large-Signal vs. Small-Signal Models The large-signal model is used to determine the DC operating point (V BE, V CE, I B, I C ) of the BJT. The small-signal model is used to determine how the output responds to an input signal.

The voltage across an independent voltage source does not vary with time. (Its small-signal voltage is always zero.) It is regarded as a short circuit for small-signal analysis. The current through an independent current source does not vary with time. (Its small-signal current is always zero.) It is regarded as an open circuit for small-signal analysis. Small-Signal Models for Independent Sources Large-Signal Model Small-Signal Model

Comparison of Amplifier Topologies Common Emitter Large A v < 0 - Degraded by R E - Degraded by R B /(  +1) Moderate R in - Increased by R B - Increased by R E (  +1) R out  R C r o degrades A v, R out but impedance seen looking into the collector can be “boosted” by emitter degeneration Common Base Large A v > 0 - Degraded by R E and R S - Degraded by R B /(  +1) Small R in - Increased by R B /(  +1) - Decreased by R E R out  R C r o degrades A v, R out but impedance seen looking into the collector can be “boosted” by emitter degeneration Emitter Follower 0 < A v ≤ 1 - Degraded by R B /(  +1) Large R in (due to R E (  +1)) Small R out - Effect of source impedance is reduced by  +1 - Decreased by R E r o decreases A v, R in, and R out

Common Emitter Stage

Common Base Stage

Emitter Follower

Ideal Current Source An ideal current source has infinite output impedance. How can we increase the output impedance of a BJT that is used as a current source? Equivalent CircuitCircuit SymbolI-V Characteristic

Recall that emitter degeneration boosts the impedance seen looking into the collector. This improves the gain of the CE or CB amplifier. However, headroom is reduced. Boosting the Output Impedance

Cascode Stage In order to relax the trade-off between output impedance and voltage headroom, we can use a transistor instead of a degeneration resistor: V CE for Q 2 can be as low as ~0.4V (“soft saturation”)

Maximum Cascode Output Impedance The maximum output impedance of a cascode is limited by r  1.

PNP Cascode Stage

False Cascodes When the emitter of Q 1 is connected to the emitter of Q 2, it’s not a cascode since Q 2 is a diode-connected device instead of a current source.

Short-Circuit Transconductance The short-circuit transconductance of a circuit is a measure of its strength in converting an input voltage signal into an output current signal.

Voltage Gain of a Linear Circuit By representing a linear circuit with its Norton equivalent, the relationship between V out and V in can be expressed by the product of G m and R out. Norton Equivalent Circuit Computation of short-circuit output current:

Example: Determination of Voltage Gain Determination of G m Determination of R out

Comparison of CE and Cascode Stages Since the output impedance of the cascode is higher than that of a CE stage, its voltage gain is also higher.

Voltage Gain of Cascode Amplifier Since r O is much larger than 1/g m, most of I C,Q1 flows into diode-connected Q 2. Using R out as before, A V is easily calculated.

Practical Cascode Stage No current source is ideal; the output impedance is finite.

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