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Op Amp LM358 somehow doesn't work

pongstark

Apr 16, 2016
1
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Apr 16, 2016
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300px-Op-Amp_Inverting_Amplifier.svg.png

Hi everyone please help. I'm using an LM358 op. amp.
1. R in =25k ohms ~ 65k ohms (flex sensor)
2. R f = 75k ohms
3. Vin = 1.6V
The problem is actually involved in the choice of Vcc.
so for the voltage supply of this circuit: I put 4.5V for Vcc, and ground the GND.
I was getting Vout = 2.81V, as I increase R in, Vout goes down a little bit (which makes sense).
But Vout should be at lease 3+V from my calculation.
Thus, I figured that my Vcc maybe is too small. Then I changed it to Vcc=9V. However, Vout is constant even though
I change R in. Vout stayed about 8V even though I didn't hook up V+ and V-.
Then I changed Vcc into 6V. Similar thing happened: Vout is constant again!!..
Obviously I do not know how to choose the right Vcc. Can someone please teach me how? And teach me how to read
this graph, please. *I'm a rookie in circuitry*
datasheet-lm358-4-638.jpg
images

(Here is the op. amp. .....and some graphs from the datasheet..... and a similar graph about voltage range [Please teach me how to read this])
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
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In the circuit you have shown gnd should be mid way between Vcc and Vee.

I assume you've tied the non-inverting input to Vee. This will cause problems.

If you note the circuit diagram on the graph you've posted, the non-inverting input is mid way between Vee and Vcc (the difference being expressed as V+, and this the voltage on the non-inverting input as V+/2

In your circuit, with the non-inverting input grounded you need Vee = -Vcc, i.e. a double-ended power supply.
 

LvW

Apr 12, 2014
604
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Apr 12, 2014
Messages
604
While reading in a forum that a simple opamp based amplifier is not working, I always assume at first that a single supply voltage is used - also in this case.
For my opinion, everybody who is trying to become familiar with opamps should at least know why classical (basic) opamp applications require a split supply because - as steve wrote - the DC operating point must be "mid way between Vcc and Vee". Otherwise, the output signal cannot swing to both directions - unless proper input DC biasing is provided.
 
Last edited:

eetech00

Nov 17, 2014
95
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Nov 17, 2014
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95
The LM358 is not rail-rail output. Max output will be about 1v less than the supply.
With the resistors you chosen, gain will be 4. Output can't go to 1.4*4=5.6v with 4.6v supply.

In addition, the opamp is configured as an inverting amplifier. So output will be inverted. With a single positive supply, output can't go negative.
 
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