1
\$\begingroup\$

I want to use LM293(comparator) to amplify signal. The input signal from 0 to 2.5V, and make the output signal double(from 0 to 5V).

Then I use the LM293, and I do the simulation first, it works fine. But when I test the circuit in reality, it doesn't works, which means when the input is 0 or something below about 1.6V, the output is always about 3.2V. When the input signal > 1.6V, the Vout= 2*Vin, which is I want.

Here is my circuit below, if you have any suggestion, please let me know. Thanks! I have draw the signal almost as I get. (I am sorry that C1 is connected between output of Lm293 and Gnd, I will update this Asap) enter image description here

\$\endgroup\$
9
  • \$\begingroup\$ Can you explain what function C1 has? \$\endgroup\$
    – Andy aka
    Apr 23, 2015 at 13:01
  • \$\begingroup\$ In order to filter the signal, I also have done test without C1. This seems no bad effect. \$\endgroup\$
    – Francisco
    Apr 23, 2015 at 13:03
  • \$\begingroup\$ You are aware that C1 is shorted out in your schematic? \$\endgroup\$
    – Andy aka
    Apr 23, 2015 at 13:08
  • \$\begingroup\$ I am sorry, the short line should be deleted. \$\endgroup\$
    – Francisco
    Apr 23, 2015 at 13:12
  • 1
    \$\begingroup\$ Why use a comparator as an amplifier? Any old op-amp would work better as a, well, amplifier. \$\endgroup\$
    – R Drast
    Apr 23, 2015 at 13:46

2 Answers 2

Reset to default
2
\$\begingroup\$

The LM293 is only guaranteed to be able to sink 6mA. You have a 100 ohm pull-up resistor and with 6mA flowing into the chip's output, the output voltage will be limited to not going below 4.4 volts. Using a typical output current sink value of 16mA, your output might be able to pull down to about 3.4 volts and you may get a shade or two lower.

If you made the pull-up resistor more like 1kohm you'd be a bit better off but, then you are going to have problems reaching the 5V rail. Either compromise your requirements, use a higher supply voltage or consider using a rail-to-rail output device.

However, even with a rail-to-rail device, the output will clip at about 4.9 volts and 0.1 volts from a single 5 volt rail.

\$\endgroup\$
1
  • \$\begingroup\$ Thanks for your suggestion. You help me figure out the problem and let me do the test later. Maybe I can change the 5V to 6V and Update the value of resistors. I have check the sink current of Lm293 from datasheet, which I miss before. \$\endgroup\$
    – Francisco
    Apr 23, 2015 at 13:28
1
\$\begingroup\$

If you must use an LM293 type comparator as a crappy low-frequency op-amp, you need to add a "BFC" (big fat capacitor) between the output and ground. Something like 10uF is probably appropriate, and I would suggest increasing that 100 ohm output resistor to a few hundred ohms at least. This adds a dominant pole to stabilize the amplifier.

This is only possible because of the simple internal structure of the LM293, and is not generally applicable to comparators (they will oscillate internally).

I also don't think you'll get very satisfactory results with a 10kHz input as you show above, but I've not done the maths.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.