Calculator Inputs
Formula Used
The calculator uses a midband small signal approximation.
Cascode output resistance: Rout = ro2 + ro1 + gm2eff × ro1 × ro2
Effective upper transconductance: gm2eff = gm2 + gmb2
Degenerated lower transconductance: gm1eff = gm1 ÷ (1 + gm1 × Rs)
Equivalent output load: Rtotal = Rout || RD || RL
Voltage gain: Av = -gm1eff × Rtotal
Decibel gain: Gain dB = 20 × log10(|Av|)
Low cutoff estimate: fL = max[1 ÷ (2πRinCin), 1 ÷ (2πRtotalCout)]
How to Use This Calculator
- Select MOSFET or BJT cascode operation.
- Enter lower and upper transconductance values.
- Enter both output resistance values.
- Add load, resistor, and degeneration values.
- Enter coupling capacitors for cutoff estimates.
- Press the calculate button.
- Review gain, resistance, bandwidth, and headroom results.
- Use CSV or PDF export for records.
Example Data Table
| Case | gm1 | gm2 | ro1 | ro2 | RD | RL | Typical Use |
|---|---|---|---|---|---|---|---|
| MOS lab stage | 6.5 mS | 8 mS | 80 kΩ | 120 kΩ | 47 kΩ | 100 kΩ | Voltage gain estimate |
| BJT small signal | 35 mS | 40 mS | 45 kΩ | 60 kΩ | 10 kΩ | 20 kΩ | Audio pre-stage check |
| Degenerated design | 10 mS | 12 mS | 90 kΩ | 140 kΩ | 33 kΩ | 68 kΩ | Linearity comparison |
Cascode Gain Design Guide
A cascode amplifier joins a lower gain device with an upper common gate or common base device. The structure raises output resistance. It also reduces Miller feedback at the input node. These effects allow high gain and wider useful bandwidth.
Why Cascode Gain Is Different
A simple common source or common emitter stage often loses gain because the output node is loaded by device resistance. In a cascode stage, the upper device shields the lower device. The lower device sees a nearly steady voltage. Current changes are sent to a high resistance output node.
The midband gain is usually estimated from effective transconductance multiplied by the total output resistance. The sign is negative for an inverting stage. The calculator keeps that sign, while also showing magnitude and decibel gain.
Important Design Inputs
The most important value is the lower device transconductance. Higher transconductance increases gain. Source or emitter degeneration lowers effective transconductance, but improves linearity. Output resistance from both devices controls the cascode resistance. The drain or collector resistor and load then reduce the usable resistance.
The upper device transconductance also matters. It multiplies both device output resistances. This creates the large cascode resistance term. For MOS circuits, body transconductance can be added when it is known. For BJT circuits, it can be left at zero.
Practical Limits
Real gain is never unlimited. Parasitic capacitance, finite load resistance, bias current, and supply voltage reduce performance. Large gain can also cause clipping when the input signal is too large. The output swing estimate helps check that risk quickly.
The low frequency estimate uses input and output coupling capacitors. It is not a full pole-zero model. It still helps compare coupling choices during early design. For final work, verify the circuit with measurement or simulation.
Because cascode stages are bias sensitive, repeat the calculation after changes. Small current shifts can move transconductance, output swing, and headroom noticeably in real hardware.
Design Use
Use this tool during lab planning, bias review, and hand calculations. Enter realistic small signal values from a datasheet, SPICE operating point, or measured curve. Then compare gain, output resistance, and cutoff results. Adjust load resistance, degeneration, and device choice until the gain target is stable.
FAQs
What is cascode amplifier gain?
It is the voltage gain produced by a stacked common source or common emitter stage and a common gate or common base stage. It depends mainly on transconductance and output resistance.
Why is the gain negative?
The lower device usually works as an inverting amplifier. A positive input change creates an opposite output voltage change. The calculator keeps this sign and also shows gain magnitude.
What does ro mean?
ro is the small signal output resistance of a transistor. Larger ro increases the cascode output resistance and may improve voltage gain when the load does not dominate.
Should I enter gmb for BJT circuits?
No. Body transconductance is mainly a MOSFET small signal term. For BJT mode, the calculator ignores gmb and uses the upper device gm value.
Why does load resistance reduce gain?
The load appears in parallel with the cascode output resistance. A smaller load lowers total output resistance. Since gain equals gm times resistance, gain falls.
What does degeneration resistance do?
Degeneration reduces effective transconductance. This lowers gain, but it can improve linearity, bias stability, and distortion performance in many practical amplifier designs.
Is the cutoff estimate exact?
No. It is a simple coupling capacitor estimate. Real circuits can include many more poles from parasitic capacitance, transistor limits, wiring, and measurement loading.
Can this replace circuit simulation?
No. Use it for quick design checks and hand calculations. Confirm final gain, bandwidth, noise, and swing with simulation, datasheet review, or bench measurement.