The Cascode Amplifier
Stack two tubes to eliminate the Miller effect, extend bandwidth by 30×, and achieve megohm-level output impedance — all without sacrificing gain.
The Cascode — Two Tubes, One Mission
Common-cathode + common-grid = wide bandwidth, high output impedance
A cascode is two amplifying stages stacked: the lower tube operates as common-cathode (input on its grid), and the upper tube operates as common-grid (its grid is AC-grounded). The signal enters the lower tube's grid and exits from the upper tube's plate.
The key insight: the upper tube's low input impedance (≈ 1/gm) clamps the lower tube's plate voltage swing. So the lower tube has a voltage gain of approximately 1. This eliminates the Miller effect almost entirely — C_Miller ≈ 2 × C_gp instead of C_gp × (1 + Av).
The voltage gain equals that of a single common-cathode stage with the same plate load — the cascode doesn't increase gain. What it does is dramatically increase bandwidth (by eliminating Miller C) and output impedance (by the factor μ of the upper tube).
Ref: Horowitz & Hill, "The Art of Electronics" 3rd Ed. §2.4.4, §3.10 — Morgan Jones, "Valve Amplifiers" 4th Ed. Ch.3
Cascode Stage Designer
Compare standard vs cascode performance with your tube parameters
Cascode Topology
Lower tube = voltage-to-current, upper tube = current buffer
The lower tube (V1) converts the input voltage to current. Its plate barely moves because the upper tube (V2) presents a low impedance (≈1/gm) at its cathode. V2 then passes this current through to the high-impedance plate load Ra, developing the output voltage. The grid of V2 is held at a fixed DC bias and bypassed to ground for AC.
Cascode Topologies Compared
Choose the right cascode for your application
| Topology | Output Z | Bandwidth | Use | Notes |
|---|---|---|---|---|
| Triode-Triode | rp × μ ≈ 6MΩ | 30× improvement | Hi-fi preamp, phono | Classic, two halves of 12AX7 |
| Triode-Pentode | 10MΩ+ | Very high | Wideband, measurement | Upper pentode gives extreme Zout |
| MOSFET-Tube | 5-50MΩ | Excellent | Hybrid designs | DN2540 + 12AX7, no heater for MOSFET |
| Folded Cascode | rp × μ | High | Low B+ designs | PNP/NPN equivalent, saves headroom |
Biasing the Cascode
Getting the upper tube bias right is critical
1. Upper Tube Grid Voltage
The upper tube's grid must be at a DC voltage that places the junction point (V1 plate = V2 cathode) at the correct operating voltage. For a 12AX7 cascode with B+=300V: set Vg2 ≈ 100–150V using a resistive divider from B+. The exact value depends on the desired operating current and plate voltage split.
2. Bypass Capacitor
The upper grid MUST be AC-grounded through a bypass capacitor. Without it, the upper tube is not a true common-grid stage, and Miller effect reduction is compromised. Use a high-quality film cap (100nF–1µF) to ground. The -3dB point should be well below 20Hz.
3. Typical Values (12AX7)
4. When to Use a Cascode
Use a cascode when: (a) you need wide bandwidth (phono preamp with 47kΩ source), (b) you need very high output impedance (for CCS-like active load behavior), (c) you want to eliminate Miller effect without switching to pentodes. Don't use when: B+ headroom is limited (you need ~150V minimum), or when you only have a single triode available.
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What is the primary benefit of a cascode amplifier?
References
- Paul Horowitz & Winfield Hill, The Art of Electronics, 3rd ed., Cambridge University Press, 2015. ISBN 978-0521809269Canonical reference for analog design — covers tubes in Ch. 2.4 & Ch. 3.
- Morgan Jones, Valve Amplifiers, 4th ed., Newnes, 2012. ISBN 978-0080966403Modern engineering treatment of tube audio design.