Extending the bandwidth down of an RF amplifier - Page 1
Forum section: electronics / projects

author: ezalys
author_rank: Frequent Contributor
timestamp: July 12, 2021, 05:41:01 am
content: Suppose you have a high performance RF amplifier from minicircuits or some such that is DC blocked around 100 kHz, and you want to pass signal down to DC. This amplifier has a 50 ohm input and output impedance. You could hack up the blocking capacitors or choose a high-speed opamp, but just out of my own edification I’m curious if there are any valid strategies for supplementing this amplifier with something else to extend the lower frequency range. Can you add another signal path for low frequencies using a high quality opamp? What else can be done?Note that this question is mostly unrelated to my previous question about low temperature tantalum capacitors. I’m just curious about amplifier design.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 12, 2021, 01:09:31 pm
content: Well, the components at work are likely in series, internally, so there's nothing beyond peaking you can do with them, externally (i.e. with a large value shunt inductor; just as you'd use a series inductor to peak node capacitance at high frequency). If you can get inside, yes, likely just increasing capacitors will do, but watch out for bias inductors as well.To get DC coupling, it might simply not be a good idea (thermal stability), but more likely consequence is increased distortion? With op-amps, you could always tap off the bias signal, add it to the input, and also servo the output voltage so it's got NFB at low frequencies, and basically normal operation at high frequencies. Any bias chokes will need to be replaced by resistors or CCS and additional supply voltage; mind the capacitance if CCS (make it into a dampened RLC circuit).Tim

author: David Hess
author_rank: Super Contributor
timestamp: July 12, 2021, 02:05:36 pm
content: Quote from: ezalys on July 12, 2021, 05:41:01 amDual path amplifiers are absolutely feasible to extend the low frequency response of an AC coupled RF amplifier.The reverse is also possible with a separate low frequency path added to create an AC coupled response in a DC coupled amplifier.

author: coppercone2
author_rank: Super Contributor
timestamp: July 12, 2021, 11:53:18 pm
content: think of it like a (active) crossover from audioyou can theoretically do that for anything that is treated as a wavethe only consequence is more noise because you have more things. if you switch unused things out of the circuit based on detectors or knowledge you improve the noise. if you power them down you save on power too.

author: bdunham7
author_rank: Super Contributor
timestamp: July 13, 2021, 12:21:35 am
content: A lot of those small RF amps are DC-biased single-ended designs that need blocking caps (or in some cases transformers) to keep their internal DC bias off of the input and output. Common designs can be as simple as a single transistor with a blocking inductor on the DC supply and two blocking caps on the input and output. Other than a complete redesign or an add-on solution like dual path, there's really not a good way to improve their low frequency performance--you would need ever larger capacitors and inductors as you go lower. OTOH, their high frequency bang-for-buck is pretty good--you can get a workable +30dB 2GHz LNA for a bit over $10.https://www.ebay.com/itm/253535286594

author: EPAIII
author_rank: Super Contributor
timestamp: July 13, 2021, 01:43:26 am
content: Analog, CRT oscilloscopes usually have input amplifiers on the channels that can be easily switch from AC coupled to DC coupled. If you want to study these amplifiers you can try downloading the repair manuals for some of these scopes and consulting the schematics. Older and high bandwidth scopes are more likely to use discrete components.Actually, digital scopes (DSOs) also have the same requirements for the analog circuitry between that input jack and the A-D converter. Yes, a DSO signal path starts with an analog amplifier. The problem there is that these circuits are often buried inside ICs and even the service manual, if you can get one, will not detail the circuitry inside those chips. High end DSOs are more likely to have input amplifiers constructed with discrete components for higher performance. Think names like Tektronix, HP, etc.You seem to have a general interest so I will mention one other way that this problem has been handles in one particular type of circuit. Analog video (NTSC, PAL, SECAM, etc.) amplifiers need frequency response from 25 or 30 Hz up to 5 or 6 Mhz. They did not need true DC response. But they also often needed to have a solid relationship between the instantaneous Voltage and some DC reference to allow different types of signal processing to take place. This is much like having a DC coupled signal without actually designing a DC coupled amplifier.The solution that was often employed was, at a point following a coupling capacitor, to clamp a known part of the signal to a known DC level. Often this clamp operated on the horizontal sync pulses (the most negative part of the composit video signal) but the front or back porch sections of the signal were also used as they separated the sync and video sections of the composit video signal. With the exception of the color burst, everything above the clamp level was video and everything below that level was sync.

author: David Hess
author_rank: Super Contributor
timestamp: July 13, 2021, 02:54:53 am
content: Quote from: coppercone2 on July 12, 2021, 11:53:18 pmNoise is not the only consequence. Mismatch in equalization between the sections spoils the transient response and increases settling time.