Preventing non inverting amplifier to generate negative output - Page 1
Forum section: electronics / projects

author: sean87
author_rank: Regular Contributor
timestamp: September 07, 2016, 07:49:26 am
content: Hi all,I need to amplify a signal in non-inverting mode by a gain of 8. I came up with the circuit below:So the gain is "1 + 2R1/R2 = 8.02" and it is fine and dandy! But since I am using an opamp "APEX MP38" according to datasheet it should be powered by both Vdd and Vss so I supply it with -40 and +40 volts.The signal comes from a 3.3v digital output of a microcontroller (pin high = 3.3 and low = 0). so when the input is high, depending on the load, I get something about +13V as output. but when the input is low or 0, I get -13 volt. So the question is, what should I change/add in the circuit to prevent it from outputing negative voltages which may damage the things on the output!

author: ebclr
author_rank: Super Contributor
timestamp: September 07, 2016, 08:08:48 am
content: You need to add another node on the + of the op ampwith a resistor to ad a DC offset , also a clamp diode on feedback cicuit

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 07, 2016, 09:02:34 am
content: If its only 8x gain opamp,why you need r2 and 3.3v bias? You only need r1 and r5 to get gain 1+r1/r5.. if you dont need -ve output,power the vss to gnd, or whatever -ve voltage necessary just to get 0v min output,not from -40v vss..

author: sean87
author_rank: Regular Contributor
timestamp: September 07, 2016, 09:09:43 am
content: Quote from: Mechatrommer on September 07, 2016, 09:02:34 amHmmm I was thinking about putting the negative rail of opamp as ground, but the datasheet of the opamp has stated minimum rails should be +/-20 so I guess in order to operate fine it needs both + and - rails!? otherwise it would blowup or something?! lol it is a 70$ opamp I cant afford to release the magic smoke!

author: MosherIV
author_rank: Super Contributor
timestamp: September 07, 2016, 09:23:48 am
content: QuoteTry with a cheaper generic op-amp first.Most Op-amps can be used in single supply rail but I cannot guarantee your Apex MP38 will.

author: tatus1969
author_rank: Super Contributor
timestamp: September 07, 2016, 09:27:20 am
content: is it just a square wave that you want to amplify? That would only require two transistors in a H bridge configuration, likehttp://www.ebay.de/itm/like/201445486277?lpid=106&chn=ps&ul_noapp=true. You don't need a 70USD power amp for that.Otherwise, if you connect +Vb=+Vs and -Vb=-Vs, then you should be able to connect your microcontroller's GND to -Vs, also connect R5 to -Vs, and consequently connect your load between output and -Vs. The input common mode range should allow this. If the equivalent circuit in the datasheet is complete, then you can even leave GND unconnected, although I would be careful with that.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 07, 2016, 09:33:43 am
content: We have been discussing on how to get larger output swing from jellybean +/- 15v $1 opamp + some bjts. you may search in this forum or google.. keyword 'Vref amplification', or 'Large output from small opamp'.i'm in office using my crappy samsung smartphone so its not easy to help you with the link. $70 opamp for only this type of app is nuts if you ask me,no offense..or if you have 30v rail,you'll just power jelly bean opamp in usual gain opamp to get 25v to 0v swing. Most jellybean accept 30v power input.

author: tatus1969
author_rank: Super Contributor
timestamp: September 07, 2016, 10:01:20 am
content: Quote from: Mechatrommer on September 07, 2016, 09:33:43 amIf he plans to use a (200V * 10A) power amp here we should assume the load to be something significant.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 07, 2016, 10:38:32 am
content: Quote from: tatus1969 on September 07, 2016, 10:01:20 amstill, it can be done in sub $10 cost with $1 opamp, few $0.2 small driving bjts, and few $1-5 power mosfets or bjts. the only reason i can think of $70 opamp is you need high power and at the same time demanding specification on regulation, ripple rejection etc. and btw. his specification is 0 - 3.3V input, 8X gain, that calculates to 26.4V max output only.

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 07, 2016, 10:52:59 am
content: here +ve rail voltage gain, you can put anything Vdd as you like as long as both bjts are specified for that voltage. opamp is ±15V input, output swing 0-Vdd, gain = 1 + R2/R1. you can adjust 5K resistor to suit your application... ymmv.

author: danadak
author_rank: Super Contributor
timestamp: September 07, 2016, 11:58:52 am
content: Something like this, attached.Regards, Dana.