# gain of op-amp

Discussion in 'General Electronics Chat' started by holden c, Jan 23, 2014.

1. ### holden c Thread Starter New Member

Jan 23, 2014
19
2
my question is about the gain of op-amp. I have the attached board and I am trying to find out if the op-amp works properly or not.

I have R1=20 k ohm, R2=200 k ohm and R3= 100 M ohm, and if I have R4= 1 G ohm resistor, I guess my overall gain should be R3/R4 for the fist stage and R2/R1 for the second stage. And total of 0.1*10=1. Therefore if I have a 1.5 Volt DC source before R4 I should be able to see 1.5 Volt at point 7. But I can not see any voltage at point 7. If I remove R4 and directly connect 1.5 V to point 2, I see 13.3 V at output (point7). I am confused about this problem. I greatly appreciate if you can help me to understand this and see if the op-amp burned out or not.

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2. ### John P AAC Fanatic!

Oct 14, 2008
1,634
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100 M ohm? 1 G ohm???

I hate to say "impossible" but you really can't use resistances of those magnitudes. I would expect empty air to be more conductive than that.

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3. ### wayneh Expert

Sep 9, 2010
12,151
3,058
This is likely as close to the rail of the power supply (which you didn't indicate) as that op-amp can produce. In other words, it's pegged full on.

Ditto on the resistor comment.

4. ### crutschow Expert

Mar 14, 2008
13,050
3,244
Few real op amps can operate with such high resistances due to input bias currents (why do you want them so high?). Generally you want to keep them below 100kΩ.

5. ### holden c Thread Starter New Member

Jan 23, 2014
19
2
thanks, my power supply is -+15 V, I think it would make sense since as I change the input voltage, output still stay the same,

6. ### LvW Active Member

Jun 13, 2013
674
100
What "makes sense"? The unrealistic resistor values?
Or the chain of two inverting gain stages with a total gain of unity?
What is the purpose of the circuit? Just an exercise?
Do you apply negative fedback (no indication of inpiut polarity)?
What kind of signal? DC or AC ? Which frequency?

As you can see: A lot of open questions.
Without proper information nobody can help you.

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7. ### holden c Thread Starter New Member

Jan 23, 2014
19
2
what made sense was by changing input voltage I did not see any change in output voltage.
the purpose of this circuit is to amplify tunneling current in a Scanning tunneling microscope (STM), in STM the order of current is nA by having bias voltage of about 1 volt which makes the resistance of about G ohm,
it is DC signal,
I am technically a chemist not a physicist so if i am not very specific about the parameters excuse me

I made the circuit work with those mentioned values on bread board, I applied 1.5 volt in and i got 1.5 volt out. But now I have another problem, when I put everything on pref board the voltage gain that I get is about 0.3 and the output voltage fluctuate, i did it couple of times and still the same result. any idea why i don't get the gain of 1?

8. ### crutschow Expert

Mar 14, 2008
13,050
3,244
If you want to amplify the 1nA tunneling current then you just need one amplifier with no input resistor and a 1G ohm feedback resistor to make a transimpedance amp. But you need an op amp with much less than 1nA of input bias current.

The perf-board likely has a resistance comparable to 1G ohm, especially if the humidity is high. To eliminate that you need to mount the summing point of the op amp and 1G resistor in air or on a teflon insulated terminal.

9. ### holden c Thread Starter New Member

Jan 23, 2014
19
2
thank you very much
can you please let me what do you mean by "summing point"? i mean where is that point?
i should mention before that R4 which is my I G ohm resistor is on another board and i just connect it with a wire to point 2 on the pref board, but everything else (R1,2,3 are all on my pref board))

i should say i don't understand most of your first paragraph!
could you please tell me what is the difference between transimpedance amp and amp? is there any ref that i can possibly read to understand more about this stuff?

10. ### holden c Thread Starter New Member

Jan 23, 2014
19
2
just one more thing that may help, when i check the output voltage of my first stage (point 1 in attached schematic) i see -0.15 V by having 1.5 V input voltage (which is normal) but i don't see 1.5 V at point 7 (i see some value around 0.5 V (fluctuates))

11. ### LvW Active Member

Jun 13, 2013
674
100
holden_c,

the series connection of two amplifiers as shown inyour first posting (why two amplifiers?) is able two amplify an input VOLTAGE.
However, later you did tell us that you want to amplify a CURRENT.
That means, you need an current-in and voltage-out amplifier (V/I=transresistance).
Such amplifiers with a (low-resistive) current input are available as integrated devices - however, as indicated in Chrutschow`s contribution you also can use a "classic" operational amplifier in transresistance configuration: Feed the current to be amplified directly into the node which connects the large feedback resistor with the inverting opamp input (that is the summing node). You must use an opamp with a very large input resistance (FET-type).

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12. ### crutschow Expert

Mar 14, 2008
13,050
3,244
The summing point is the negative (-) input of the op amp. That point and anything connected it to that point must go only to a very high resistance insulator such as air or a teflon insulator for the low currents you want to measure. Long leads to that summing point should be avoided to minimize noise pickup. The 1G ohm resistor should be mounted close to the op amp.

A transimpedance amplifier is typically an op amp connected to directly convert current to voltage (see here). You feed the current directly into the summing junction (no input resistor) with a feedback resistor (in your case 1 G ohm) between the output and the summing junction. That will give an output voltage of 1V for a 1nA input current.

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Jan 23, 2014
19
2

14. ### LvW Active Member

Jun 13, 2013
674
100
Voltage gain? Which input?
I think, now we are speaking about a current-in amplifier?

15. ### crutschow Expert

Mar 14, 2008
13,050
3,244
If you are measuring current then there's no reason for the first op amp (if it's still in the circuit). That's measuring voltage, not current. To measure current all you need is the second op amp and the 1GΩ resistor.

16. ### holden c Thread Starter New Member

Jan 23, 2014
19
2
right, my bad, yes we are talking about a current in amplifier, my mean by input was the current i am feeding into point 2.

17. ### holden c Thread Starter New Member

Jan 23, 2014
19
2
crutschow,
i know you get frustrated! but could you please explain to me why my first op-amp measures voltage and the second one measures current?