| Electronics > Beginners |
| Exponential Amplifier |
| (1/6) > >> |
| eev_carl:
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 |
| HB9EVI:
try just with the diode on the inverting input, but with the cathode connected to it |
| David Hess:
Where did you find that example? The transistor and diode are backwards; the emitter of the transistor is on the input side. |
| exe:
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). |
| David Hess:
--- Quote from: exe on July 24, 2018, 08:57:55 am ---Common workarounds are to digitize the signal and process in digital domain. --- End quote --- That 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. |
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