Exponential Amplifier - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: July 23, 2018, 02:30:51 pm
content: Hi,I built an exponential amplifier but the circuit doesn't seem to respond to the changing Vin. The schematic is attached. I'm using a PN2222A, 741, and 8148.When I vary the Vin from 10mV-100mV-1V-10V, my Vout remains at 1.5mV.Thanks in advance,Carl

author: HB9EVI
author_rank: Frequent Contributor
timestamp: July 23, 2018, 02:51:35 pm
content: try just with the diode on the inverting input, but with the cathode connected to it

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2018, 02:04:06 am
content: Where did you find that example? The transistor and diode are backwards; the emitter of the transistor is on the input side.

author: exe
author_rank: Unknown
timestamp: July 24, 2018, 08:57:55 am
content: Just in case, all analog exponential / log amplifiers I know are quite imprecise (like, drifting with temperature, etc). Those based on diodes drift the most, better ones use BJTs (still not ideal). There are articles on the subject explaining why. Common workarounds are to digitize the signal and process in digital domain. Or approximate log/exp scale with multiple amplifiers (each with its own fixed gain).

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2018, 11:08:53 am
content: Quote from: exe on July 24, 2018, 08:57:55 amThat must be fun when the dynamic range exceeds any ADC ever made.Accuracy of better than 1% is feasible with proper temperature compensation or if the temperature of the translinear element is fixed.

author: eev_carl
author_rank: Regular Contributor
timestamp: July 24, 2018, 01:30:49 pm
content: Quote"The Forrest Mims Circuit Scrapbook Vol 1."I build a logarithmic amp from the same article and its transfer function worked perfectly. I've seen different schematics with the diode reversed too.

author: eev_carl
author_rank: Regular Contributor
timestamp: July 24, 2018, 01:32:27 pm
content: QuoteThis isn't for a practical application. I have ADCs, MCUs, and DACs here too.

author: eev_carl
author_rank: Regular Contributor
timestamp: July 24, 2018, 01:38:08 pm
content: QuoteI've tried this too, but am having trouble figuring out the transfer function. If I just run a diode into the -V of the amp, what would the values look like? The input needs to be >0.7V to activate the diode, right?This is just for a demo so I'm not concerned with errors from temp, etc. I'm looking for something that I can plot with a rough exponential curve.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: July 24, 2018, 02:03:41 pm
content: it should look like an antilog curve in the x-y coordinates.maybe stupid to ask, but are you running the opamp with a bipolar supply? you know, it's actually an inverting circuit, so the output goes to the negative rail of the supply; the circuit cannot work from a single supply

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 24, 2018, 02:03:51 pm
content: Quote from: David Hess on July 24, 2018, 02:04:06 amExactly. It is just an error in the book.CheersAlex

author: eev_carl
author_rank: Regular Contributor
timestamp: July 24, 2018, 02:08:39 pm
content: QuoteIt's split. I threw in a resistor instead of the diode and ran in a sine wave to verify the 741.

author: eev_carl
author_rank: Regular Contributor
timestamp: July 24, 2018, 02:25:18 pm
content: I reversed the BJT and get these outputs. It seems like i might not be operating in the correct range. Maybe I'm missing an offset? Nothing interesting happens until about 10V.VinVout10mV1.5mV100mV1.5mV1000mV1.5mV10000mV22.6mV

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 24, 2018, 02:30:31 pm
content: Quote from: eev_carl on July 24, 2018, 02:25:18 pmThe input should be negative polarity... and please limit the current on the input by, say, 10mA.CheersAlex

author: eev_carl
author_rank: Regular Contributor
timestamp: July 24, 2018, 02:33:51 pm
content: Here's what I get with negative polarityVinVout-10mV1.5mV-100mV1.7mV-1000mV8.22V-10000mV8.16VThe current is >5mA for Vins >1V. Should I put a 220ohm resistor in series?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 24, 2018, 02:53:18 pm
content: Quote from: eev_carl on July 24, 2018, 02:33:51 pmIt could be too late for it - if you've really managed to apply 1V and 10V voltage to the input... .CheersAlex

author: eev_carl
author_rank: Regular Contributor
timestamp: July 24, 2018, 03:20:18 pm
content: QuoteWhy? Isn't the 741 good for +-15V and the PN2222A good for 50V or so?

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2018, 04:50:54 pm
content: Quote from: eev_carl on July 24, 2018, 03:20:18 pmThe transistor's emitter-base breakdown voltage is only about 5 volts which is why the diode is included in the circuit. Emitter-base breakdown is very damaging to transistors which are intended for use in translinear circuits.For now why not use just a diode instead of the transistor to get an idea of what should be going on?And move the capacitor to be in parallel with the feedback resistor. Where it is now causes the operational amplifier to act as an unstable differentiator. The circuit as shown even with the transistor and diode used correctly will oscillate.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: July 25, 2018, 01:23:51 am
content: may it help:https://www.ti.com/ww/en/bobpease/assets/AN-31.pdf

author: nugglix
author_rank: Regular Contributor
timestamp: July 25, 2018, 09:51:42 am
content: Newer version, revised 2013:http://www.ti.com/lit/an/snla140b/snla140b.pdf

author: eev_carl
author_rank: Regular Contributor
timestamp: July 25, 2018, 01:21:05 pm
content: Hi,I built the exponential amp w. a single diode. Here is my schematic and test results. The results produce a straight line on a semi-log graph which is what I was looking for.Thanks to all(in mV)VinVout200-0.7300-13.6400-109.6500-828600-4590

author: exe
author_rank: Unknown
timestamp: July 25, 2018, 01:29:08 pm
content: Quote from: eev_carl on July 25, 2018, 01:21:05 pmImho this can't work. But even if does, it's just wrong. Diode is biased only by opamp's parasitic input current, that's not how to build circuits.

author: eev_carl
author_rank: Regular Contributor
timestamp: July 25, 2018, 01:33:15 pm
content: QuoteWhat do you think about the anti-log schematics in the TI links posted above? Are you suggesting that the diode should be replaced with a BJT?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: July 25, 2018, 01:38:54 pm
content: Quote from: eev_carl on July 25, 2018, 01:21:05 pmYour schematics is wrong. It is not what you've built and measured.CheersAlex

author: eev_carl
author_rank: Regular Contributor
timestamp: July 25, 2018, 01:42:16 pm
content: QuoteThat's my bad Eagle skills...fixing + to ground(replaced incorrect Eagle schematic with handdrawn schematic showing the inverting amp)

author: exe
author_rank: Unknown
timestamp: July 25, 2018, 02:08:56 pm
content: Quote from: eev_carl on July 25, 2018, 01:33:15 pmAh, now I see the updated schematics. I'd say, if diode works for you, then don't make your life harder with more complicated circuits. Just know limitations of your solution (mainly thermal drift).

author: David Hess
author_rank: Super Contributor
timestamp: July 25, 2018, 07:51:59 pm
content: Quote from: exe on July 25, 2018, 01:29:08 pmIt will work fine and I have done it this way many times in uncritical applications. The input bias current of the operational amplifier limits accuracy at low currents but so does the diode's leakage which can be considerable if a gold doped switching diode like the 1N4148 is used.A transistor offers much better dynamic range but most applications do not require it.

author: eev_carl
author_rank: Regular Contributor
timestamp: July 25, 2018, 07:53:51 pm
content: QuoteThis might have referred to a bogus Eagle schematic I put up with the terminals of the op amp mixed up. My handdrawn schematic is what I have on the breadboard.