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Plate Resistance
On Jun 2, 10:01*pm, Roy Lewallen wrote:
This might shed a little light on the discussion. Consider a diode, forward biased by a 10 mA DC current source connected directly across it. The characteristics of this particular diode are such that the voltage is exactly 0.7 volt. .... The lengthy discussion about tube plate resistance has muddled the DC operating point (equivalent to the diode DC bias) and the plate resistance, which is the plate's AC or dynamic resistance. They're related just the same as for the diode. And just like the diode, if you were to send an AC signal to the plate of a biased tube through a capacitor, you'd find an AC voltage and current which are in phase resulting in power being delivered to the tube, and an increase in plate dissipation in the amount of that power. Roy Lewallen, W7EL I'm happy to see that Roy posted this clarification about dynamic versus static resistance. I haven't read all the posts here, but those others that I have seemed to completely skirt the issue of the meaning of "plate resistance." Further to Roy's posting, though, you must realize that a device with more than two terminals is complicated by the fact that you need to specify that the dynamic resistance is measured with other terminals held constant. So, for example, plate resistance of a tube is normally defined as the partial derivative of plate voltage with respect to plate current, with the grid-to-cathode voltage held constant, and of course at some particular plate voltage (and corresponding current). For a tetrode, the screen-to-cathode voltage must also be constant. The reason for this is that you will get completely different answers if you don't hold the voltage of the grids constant with respect to the cathode. For example, if you set up a circuit with a tube with the grid grounded, and an appropriate resistor between the cathode and ground (not bypassed), to establish a desired bias point, then a change in plate current will be accompanied by an essentially identical change in cathode current, and a corresponding change in drop across the cathode resistor, which represents a change in grid-to- cathode voltage. Measuring dV/dI at the plate in such a circuit will yield a much higher resistance than if the grid-to-cathode voltage is kept constant. It is, in fact, a good way to make a constant current source. There is no particular relationship that must be maintained between the load resistance you present to an amplifier element -- a tube or a bipolar transistor or a FET -- and the plate/collector/drain (dynamic) resistance of the device. For example, it's common to operate a push- pull pair of 6146's in audio service with a load around 5000 ohms (plate to plate) at 500V plate supply voltage, 185V screen voltage, and perhaps 30mA plate current. That's effectively 2500 ohms to each plate of the pair. But at that operating point, the plate resistance of a 6146 is around 600V/30mA, or 20k ohms (roughly, from plate characteristic curves in my old RCA transmitting tubes manual). On the other hand, a triode will have a much lower plate resistance, likely similar to the load resistance--but again, not really related to it. Another rather interesting thing happens, though, when you connect an RF load through a tank circuit such as a PI network. It's quite possible for a pi network to transform a 50 ohm RF load so it presents a reasonable (say) 4k ohm RF load to the plate of a tube like a 6146. But that same pi network will in turn transform the plate resistance-- or rather, the net impedance of the tube in its particular circuit configuration (grounded grid or grounded cathode, and invariable some amount of feedback whether you wanted it or not), along with the DC feed (RF choke) in parallel, to quite possibly an impedance not very close to 50 ohms, and likely rather reactive. I can, without much difficulty, design a PI network that will yield a source resistance at the output connector that's a lot less than 50 ohms, using a tetrode amplifier tube, while giving a very decent transformation of a 50 ohm load to a desired load at the plate of the tube. And as if that weren't enough, I can modify the output impedance further by application of feedback; this is more commonly done at audio frequencies, where amplifiers designed to drive loads like 4 ohms an 8 ohms have output impedances in the area of a small fraction of an ohm. But it's also done with RF amplifiers, often for reasons unrelated to the output impedance, but also to control the output impedance so that it IS very close to 50 ohms (for example in instrumentation, where it may be important). Cheers, Tom |
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