In terms of raw bandwidth, general-purpose transistors are usable up into the 10s of MHz. If the baseline comparison is an average 1MHz op-amp like LM358, then there is at least a little merit to considering discretes. But it may be more interesting to explain what "good" is, and why this is the case. I won't go into detail of possible discrete solutions here needless to say, the design space is vast, and. At that voltage level, slew rate will be the bigger limitation a JFET type would probably do nicely. (With dual diodes being essentially no added cost, this is an easy win these days.)Ī few MHz is not very high, and an op-amp of some 20MHz+ GBW will do, readily available in western markets. Thus, the lower diode is not strictly necessary for basic function, but improves switching speed. ![]() This incurs an apparent recovery delay due to the op-amp's limited slew rate (also called integrator windup). There's also the variation with a single diode (the top one), which has the disadvantage that IC1B's output is unconstrained and therefore saturates while the input is positive. The diode should have low capacitance, and lower impedances/resistor values can be chosen to deal with the diode capacitance (BAV99 or BAS70 would be better than BAT54 generally). Performance (bandwidth and offset/balance) depends largely on the op-amp chosen. Performance at higher carrier frequencies may be explored at a later date. The solution has an accuracy of within 0.5% for a signal range from 10% to 100%, and within 1% for signal range from 5% to 100%, for a carrier of 1MHz. A solution was developed via LTspice, using common BJTs, which met the criteria once the requirements regarding zero-signal offset were relaxed. Update 2023-Jul-29 Apologies for the delay in responding, I was diverted to other activities over the past few weeks. I will post my findings when they are available. I have a response below that is my attempt at collating the images for solutions that did not have images originally in this post (& its subsequent answers). My preference is for solutions that use standard speed op-amps (1MHz, 1V/us) and jelly-bean parts. I have started collecting possible solutions and will attempt to evaluate them over the next few days. Note 3 Updates (after Original Question posted) The output of the detector is to be 0V to 5V (0% to 100% of input) with linearity within 1% and offset < 1%. Update: this article with schematic was found, refer posts below, my sincere thanks to Tampieri, and can anyone suggest a simple circuit that does not use more than two (2) op-amps to achieve this? The output can be either full-wave (absolute value) or half-wave (precision rectifier,) preferably operating from a single supply rail of +12V, which unfortunately precludes the rather nice solution offered by the TI app note TIDU030, as explained in Note 3. Mancini in its original form, or some other source that shows the schematic. I would be grateful if someone could furnish this article from Mr. To my dismay, Figure 1 is not available - I even tried the Wayback machine without success. This circuit's output voltage is an accurate representation of the absolute value of the input signal, and it is accurate for input signals containing frequencies as high as 10 MHz. The circuit in Figure 1 uses three fewer components than most absolute-value circuits require, and only two of the resistors must have 1% tolerance to obtain 1% accuracy. I found an article dated 1 by Ron Mancini titled: " Absolute-value circuit delivers high bandwidth" (Note 2, below), the 2nd & 3rd sentences of which seem to offer everything I seek: ![]() ![]() The bandwidth of the detector output is not that high, about 1kHz. Op-amp-based precision rectifiers (refer Note 1, below) may be suitable, but the relatively high frequency of 1MHz may prove challenging for typical opamps having GBWP~2MHz & slew rate ~1V/us. It sounds easy enough, but it needs to measure the signal accurately when the amplitude is just 1% of full-scale (0.1V peak) so the usual diode detectors are simply not suitable due to diode Vf being ~0.5V. The signal at full-scale is quite large, being a 1MHz sine-wave, with zero-offset and peak-to-peak amplitude of 20.00V (+10V, -10V peaks). ![]() I need to detect (convert to DC, or low frequency <1kHz) the amplitude of a signal.
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