Current Mirrors with Tubes
Forcing one tube to copy the current of another — precision biasing without resistors
A current mirror uses a reference tube (diode-connected) to set a current, then forces a second tube to replicate it. The reference tube's grid-cathode voltage V_gk is applied to the mirror tube's grid, producing the same plate current (assuming matched tubes). In the semiconductor world this is trivial; with tubes it requires careful attention to matching and compliance voltage.
The accuracy depends on: (1) tube matching — gm and μ must be close, (2) Early voltage analog — finite rp causes output current to vary with plate voltage, (3) temperature — both tubes must be at the same temperature (dual triodes are ideal). With a 6SN7 dual triode, matching within 5% is typical; hand-selected pairs can achieve 1-2%.
Current Mirror Design
Compare simple, cascode, and Wilson mirror performance
Mirror Topologies
Mirror Topologies Compared
From simple to precision — choosing the right current mirror
| Type | Tubes | Z_out | Accuracy | Use case |
|---|---|---|---|---|
| Simple | 2 | rp | 5-10% | Basic CCS, tail current |
| Cascode | 3 | μ×rp | 2-5% | High-Z loads, diff pair tails |
| Wilson | 3 | μ×rp/2 | 1-3% | Precision current sources |
| Widlar | 2 | rp×(1+gm×Re) | 3-5% | Sub-mA currents with cathode R |
| MOSFET hybrid | 0 | > 1MΩ | < 1% | Modern designs, DN2540/LM334 |
Building Tube Current Mirrors
1. Tube Selection
Use dual triodes (6SN7, 12AU7, 6922) for best thermal matching. The two sections share the same envelope temperature, reducing drift. For higher currents, the 6BX7 (dual power triode, 15W per section) works well. For precision, the 6SN7 is the gold standard — low rp, high gm, excellent linearity.
2. Setting the Reference Current
The reference current is set by a resistor from B+ to the diode-connected tube's plate: I_ref = (B+ - V_ak) / R_ref. For a 6SN7 at 5mA, V_ak ≈ 80V, so with B+ = 300V: R_ref = 220V/5mA = 44kΩ. Use a 47kΩ resistor (standard value) for I_ref ≈ 4.7mA.
3. Improving Accuracy
Add small cathode degeneration resistors (100-500Ω) to both tubes. This reduces sensitivity to gm mismatch at the cost of lower output impedance. For the cascode version, add a second tube above the mirror tube with its grid at a fixed voltage — this multiplies Z_out by μ and shields the mirror from plate voltage variations.
4. Modern Hybrid Approach
In modern tube designs, MOSFET current sources (DN2540, IXCP10M45S) or solid-state references (LM334) often replace tube mirrors. They offer better accuracy (< 1%), lower minimum voltage, and no matching concerns. The purist may object, but the electrons don't care — the performance improvement is real and measurable.
Testez vos connaissances
In a simple tube current mirror, what sets the mirror current?
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.