Amplifier Topologies
Four fundamental building blocks of tube amplification. Interactive calculators, SVG schematics, and every formula you need — with live results as you adjust parameters.
Common Cathode
The workhorse of tube electronics. Maximum voltage gain, inverted output, the default topology.
The common cathode amplifier is the tube equivalent of a common-emitter transistor stage. Signal enters the grid, the cathode is grounded (AC), and the amplified, phase-inverted signal appears at the plate.
The plate resistor Ra converts the tube's varying current into a voltage swing. Higher Ra = more gain, but less headroom. The cathode resistor Rk sets the DC bias.
With Ck (bypass capacitor), full gain is achieved. Without it, local negative feedback through Rk reduces gain but dramatically improves linearity — a trade-off used constantly in hi-fi design.
Cathode Follower
Unity gain, very low output impedance, non-inverting. The impedance transformer.
The cathode follower takes the output from the cathode instead of the plate. The plate connects directly to B+. The result: gain slightly less than unity, but with extremely low output impedance — approximately 1/Gm.
This is 100% negative feedback: the entire output signal is fed back to the input. The tube cancels its own distortion, producing an exceptionally linear transfer function.
Use it between a high-impedance source and a low-impedance load: driving long cables, tone stacks, headphones, or the grid of a power tube that draws current.
SRPP
Two triodes stacked: one amplifies, the other regulates. Gain + low impedance in one stage.
SRPP (Shunt Regulated Push-Pull) stacks two triodes vertically. The lower triode (V1) is a common cathode gain stage. The upper triode (V2) acts as a dynamic load — part constant current source, part push-pull partner.
On positive signal swings, V1 conducts more and V2 less. On negative swings, the opposite. The result: push-pull operation from a single-ended input, with partial cancellation of even harmonics.
The magic happens at a specific load impedance (RL ≈ 2 × rp). At this value, distortion cancellation is maximized. Too light or too heavy a load, and the SRPP degenerates into an ordinary common cathode with resistive load.
Mu Follower
A constant current source as plate load. Maximum gain approaches μ itself.
The mu follower replaces the plate resistor with an active constant current source — the upper triode (V2) with its grid held at a fixed bias. This CCS has an effective impedance of rp × (μ+1), which for a 12AX7 is 6.3 MΩ — impossible to achieve with a real resistor.
Because gain = μ × Ra / (rp + Ra), and Ra is now millions of ohms, the gain approaches the tube's theoretical maximum: μ itself. A 12AX7 mu follower delivers gain ≈ 99 (vs ~60 with a 100kΩ resistor).
The output is taken from the cathode of V2 — a cathode follower output with very low impedance. So you get near-maximum gain AND low output impedance. The best of both worlds.
Topology Comparison
Side-by-side characteristics of all four topologies.
| Parameter | Common Cathode | Cathode Follower | SRPP | Mu Follower |
|---|---|---|---|---|
| Gain | High (−μ·Ra/(rp+Ra)) | < 1 (unity) | Medium-High | Very High (≈ μ) |
| Phase | Inverted (180°) | Non-inverted (0°) | Inverted (180°) | Inverted (180°) |
| Output Z | High (Ra ∥ rp) | Very Low (rp/(μ+1)) | Low (2rp/(μ+1)) | Very Low (rp/(μ+1)) |
| Input Z | High (≈ Rg) | Very High (boot.) | High (≈ Rg) | High (≈ Rg) |
| Linearity | Good (with NFB) | Excellent | Very Good | Excellent |
| Tubes needed | 1 (½ dual) | 1 (½ dual) | 2 (1 dual) | 2 (1 dual) |
| Best for | Voltage amp | Buffer / driver | Headphone amp | High-gain line |
| Sonic character | Direct, articulate | Transparent, open | Fast, dynamic | Lush, detailed |
Choosing wisely: The common cathode is your default voltage amplifier. Add a cathode follower when you need to drive a difficult load. Use SRPP when you need gain + low impedance in one envelope (headphone amps). Choose the mu follower when transparency and maximum resolution are the priority — it's the most refined topology, favored by the world's best line stage designers.
Every Formula You Need
Organized by topology and application. Bookmark this page.
Design Rules of Thumb
Shortcuts that experienced designers carry in their heads.
Test Your Knowledge
Review the key concepts of tube amplifier topologies covered in this guide.
What is the key characteristic of a common cathode amplifier?