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Why is the bandwidth of pentode amplifier larger than the bandwidth of triode amplifier?

I have searched the following books but can't find the answer:

  • Electronics devices and circuits by Rajiv Tiwary
  • Electronics devices circuit and theory by Robert L Boylestead
  • Allen Mottershead
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  • \$\begingroup\$ I don't even think that's true, in general. It's just that pentodes hit the market when demand for higher bandwidths appeared. \$\endgroup\$ – Marcus Müller Mar 15 at 12:54
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    \$\begingroup\$ @MarcusMüller The screen grid in tetrodes and pentodes is usually decoupled, reducing the grid-anode coupling capacitance and thus the Miller effect. That has a huge effect on the bandwidth. Its importance today (apart from the vanishingly rare and getting harder to find tetrode MOSFETs) is its inspiration for the cascode configuration, which does the same. \$\endgroup\$ – Brian Drummond Mar 15 at 13:05
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    \$\begingroup\$ Context of that assertion may be important. I'd considered the Miller capacitance (plate -to- grid) to be the deciding factor. Many high-frequency amplifiers used grounded-grid configuration. Others went to the trouble of neutralizing plate-to-grid capacitance of triodes. \$\endgroup\$ – glen_geek Mar 15 at 13:06
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    \$\begingroup\$ @MarcusMüller It's interesting to read, partly how concepts can come round again, like tetrode -> cascode. And how many have no modern semiconductor equivalent. Change screen grid geometry, and you can vary the transconductance by varying screen grid voltage - the vari-mu valve is much simpler than our analog multiplier (the BF981 or 3N140 came pretty close though) \$\endgroup\$ – Brian Drummond Mar 15 at 13:27
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    \$\begingroup\$ Please edit your post with links or book references to substantiate your claim that pentode amplifiers have higher bandwidth. We can't explain someone else's statement if we don't see it. \$\endgroup\$ – TimWescott Mar 15 at 14:40
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Bear with me, the schematic editor wasn't really made for this...

schematic

simulate this circuit – Schematic created using CircuitLab

In the circuit on the left, we can see 10pF feedback capacitance, carrying a fraction of the anode voltage back to (and in antiphase with) the input voltage on the grid. This capacitance arises naturally from two conductors in close proximity. As the anode voltage is many times the input voltage, this practically destroys the gain at high frequencies.

The circuit on the right depicts a tetrode valve, a natural progression towards the pentode. (I can't find a good way to depict a suppressor grid, and FETs don't generally have secondary emission problems so there's no need for one).

In this, GRID 2 is biased to a DC voltage, therefore the Miller charge is conducted harmlessly to ground (in an AC analysis we ignore the DC bias voltages), and GRID 1 sees only 10pF to GRID 2 = ground.

As the second grid is an open structure (a spiral of very thin wire) there IS still some remaining Miller capacitance from anode to grid 1 - the value here ( 7 Femtofarads) is taken from a 1940s EF50 pentode.

This has much less effect on the bandwidth than the triode's 10 pF.


Now this is very much a 21st century problem, since we have progressed from 10pF or so Miller capacitance in a typical vacuum tube triode, to 2000pF or so, in a power MOSFET. When you turn a MOSFET switch on, as the anode(cough)drain voltage starts to fall, the grid(cough)gate voltage flattens for many nanoseconds, as you pump in current to counter the miller capacitance. This is a well known issue in SMPS, motor drivers, inverters etc, requiring amps of gate drive current to switch fast and reduce switching losses.

If somebody reading this could see a practical way of tetroding a MOSFET switch without wasting power in the cathode to Grid2 voltage, that would make for a pretty valuable patent!

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  • \$\begingroup\$ Thanks for this discussion. I learned electronics on vacuum tubes, as it was all I could really get my hands on when growing up. (Transistors were impossibly expensive or simply unavailable.) I had to struggle long times gathering the physical details of triodes, tetrodes, and pentodes. (The grid-leak self-bias resistor was one of my "hurdles" I found an early, difficult barrier to cross when starting out as I couldn't seem to apply Ohm's law to it.) Nice answer. \$\endgroup\$ – jonk Mar 15 at 17:42
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consider the RCA 3N170 dual-gate MOSFET, of era 1970s.

Miller Effect was greatly reduced, and the precious RF energy could be used in lower-capacity-higher-inductance high-Q narrow-bandwidth amplifiers that had many dBs more gain.

And, as others indicate, the S-param S12 and S21 change dramatically, usefully altering the stability-circles.

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