ceramic potentiometer maximum frequency? - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: August 27, 2017, 07:53:36 am
content: I am making an amplifier capable of going to 20MHz. I want to have a gain adjust on it.Will a ceramic potentiometer work directly in the feedback loop?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 27, 2017, 08:35:19 am
content: It depends on the resistance. With high resistance the parasitic capacitance is too high and will cause trouble.A pot in the feedback path is also sometimes tricky as the inverting input of an OP (or similar amplifier) may be sensitive to extra capacitance. A pot tends to have higher capacitance than fixed resistors.A pot as a divider is usually a little less critical.

author: Zero999
author_rank: Super Contributor
timestamp: August 27, 2017, 10:03:51 pm
content: Quote from: Kleinstein on August 27, 2017, 08:35:19 amAnd at low resistances, the parasitic inductance can be a problem. Although inductance shouldn't be an issue at 20MHz, especially at lower power levels.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 28, 2017, 01:06:59 am
content: Large trimpots and small potentiometers (1W or more?) should be fine at 20MHz.Small SMT trimpots are reasonable up to a few GHz, for ~Zo values (say 20 to 200 ohms).The ratio to Zo is important. That sets whether the element will behave capacitive or inductive beyond the lower cutoff frequency, and where that frequency will fall.Tim

author: David Hess
author_rank: Super Contributor
timestamp: August 28, 2017, 03:07:04 am
content: As long as you do not pick an unreasonable impedance level, circuit topology, or adjustment range, then even a large potentiometer will work to 20 or even 50 MHz. Old oscilloscope vertical amplifiers inserted the variable gain potentiometer into the cathode/emitter side of a transconductance amplifier. At higher frequencies it would have to be part of the anode/collector load where the impedance is higher.Also take a look at how old 20+ MHz function generators used potentiometers to control their output level.

author: KE5FX
author_rank: Super Contributor
timestamp: August 28, 2017, 04:53:09 am
content: A small ceramic trimpot can work well at RF. Instead of putting it in a feedback loop, though, where both ends will be 'hot', consider a non-inverting stage with a CFB opamp, where the gain adjustment pot goes from the - input to ground. This topology can provide stable gain control greater than +15 dB at 100 MHz or so.I keep a few of these around the lab in Hammond boxes, powered from NiMH rechargeables:

author: CopperCone
author_rank: Super Contributor
timestamp: August 28, 2017, 05:36:34 am
content: Ah, problem with that is that its a DC to 20MHz amplifier.Why do you tie it to -5v instead of gnd?

author: KE5FX
author_rank: Super Contributor
timestamp: August 28, 2017, 06:04:51 am
content: Quote from: CopperCone on August 28, 2017, 05:36:34 amThe 3-dB bandwidth at the +15 dB gain setting is about 110 MHz. That's the nice thing about CFB opamps; they don't have a GBW product per se. There is some additional rolloff at gains above 15 dB but I imagine it's due to parasitic L and C as much as anything else.QuoteThese amps need a lot of output headroom, since they aren't rail-to-rail capable. You lose about 1V at the rails and half of what's left in the series termination that's required to drive 50-ohm coax, so the bipolar supply is actually pretty important.

author: David Hess
author_rank: Super Contributor
timestamp: August 28, 2017, 03:43:22 pm
content: Quote from: KE5FX on August 28, 2017, 06:04:51 amThe gain-bandwidth product of a current feedback operational amplifier is set by its feedback resistance which controls the current available for charging the internal compensation capacitance. This is similar to uncompensated voltage feedback operational amplifiers where their gain-bandwidth product may be adjusted to fit the closed loop gain by adjusting the compensation. The difference is that current feedback amplifiers have a lot more current available while in voltage feedback amplifier, the current is limited by the differential input stage tail current. (1)If the potentiometer was arranged so that the feedback resistance decreased with increasing gain, that would increase the current available and raise the gain-bandwidth product as the closed loop gain increases so closed loop bandwidth would be maintained at higher gains.(1) If the input stage current is increased, then the transconductance is increased and the amplifier becomes unstable anyway which is why fast voltage feedback amplifiers use all kinds of tricks to lower the transconductance of the first stage like adding emitter resistors (LM318 and video amplifiers), using FETs (FETs have a lower transconductance at a given source current than bipolar transistors), or transconductance reduction (used to make the MC1556 fast and the LM324 inexpensive).

author: KE5FX
author_rank: Super Contributor
timestamp: August 28, 2017, 07:17:36 pm
content: Quote from: David Hess on August 28, 2017, 03:43:22 pmEach part is optimized for a particular feedback resistance (237 ohms in the case of the LMH6702.) You really do not want to put potentiometers, capacitors, or anything else besides the recommended resistance in the feedback path. Good (i.e., bad) way to build an oscillator.