Voltage amplification

[gtrplaya101]

Ok so I need a little help with a circuit i'm designing. Here's my problem, I have a IC that is giving me a logical output of about 2-3v, however I need that to be around 6 to 7 volts. Should/Can I use an op-amp or transistor to get this done. Which would be easier? Thanks Curt

[anchovie]

If your circuit has a 9v supply, you could use an inverting buffer rather than an amplifier. Your 2-3v will get subtracted from the 9v, giving you a very convenient 6-7v!

[gtrplaya101]

I have a couple of CD4069 laying around you think that would work?

[anchovie]

No, the 4069 is a logic chip so in this case it would either be high (supply voltage) or low (ground) and nothing inbetween.

I'd use an op-amp: connect your 2-3v to the inverting input with a 10k series resistor, connect the output to the inverting input with another 10k resistor and connect the non-inverting input to ground.

[gtrplaya101]

Will this supply a output voltage even if my chip's output drops to zero? If the chips output drops to zero I also need the amplified voltage to drop to ground.

[anchovie]

Ah, there's a problem. The inverting buffer would then put out the same as the supply voltage.

Is this 2-3v potentially going to vary between that range, or will the voltage be somewhere between those two values but not move? If it varies, that's what makes the amplification idea not so simple - you could have a x3 amplifier to turn 2v into 6v, but if 3v goes into it you get 9v out rather than 7v. If the voltage is going to be stable and repeatable then you can just set up an op-amp to multiply it by the required amount.

[gtrplaya101]

Thanks for your help this far!
Ok i just measured the output and it is reading steady at 2.0v so i need what about 3.5x amplification?

[anchovie]

If you get an op-amp and connect a 10k resistor from the inverting input to ground, a 22k resistor from the output to the inverting input and connect your logic signal to the non-inverting input using a 6.8k series resistor you'll have a non-inverting amplifier with a gain of 3.2, which should give you a nice 6.4v on the output.

[gtrplaya101]

I can use say a lm386 correct where I don't need a bypolar power supply correct?

[anchovie]

The LM386 is an audio power amplifier rather than a standard op-amp. It's not the ideal choice.

You can have a single supply if you use an op-amp such as a TLC271 - with this chip the output voltage swing includes the negative voltage rail so 0v in will give 0v out. Of course any rail-to-rail output op-amp will be ideal as well.

[German]

Voltage amplification - transformer 

[gez]

If all you want is logic out at a higher voltage, use a level-shifting circuit.  In other words a switch.  Connect the low logic's output to a transistor's base via a resistor, wire said transistor's emitter to ground and stick a resistor from its collector to the higher voltage.  The collector provides a logic output at your new (higher) voltage.  Be aware of loading issues.  If your higher supply is 9V rather than the 6-7 you require, then your going to need to drop some voltage somewhere.

Personally, I wouldn't use op-amps.  More parts/work plus many are unsuitable for the task (don't like inputs being pulled to ground). 

[gtrplaya101]

If all you want is logic out at a higher voltage, use a level-shifting circuit.  In other words a switch.  Connect the low logic's output to a transistor's base via a resistor, wire said transistor's emitter to ground and stick a resistor from its collector to the higher voltage.  The collector provides a logic output at your new (higher) voltage.  Be aware of loading issues.  If your higher supply is 9V rather than the 6-7 you require, then your going to need to drop some voltage somewhere.

Personally, I wouldn't use op-amps.  More parts/work plus many are unsuitable for the task (don't like inputs being pulled to ground). 

I agree a transistor would be easier but this wiring does not make sense to me. Shouldn't the lower voltage source be wired to base. 9v wired to collector and emitter wired to where I need the higher voltage?
Actually just figured out these are basically the same thing. What value resistors would I need or how do I calculate it if I am looking to get the transistor into what the saturated region?

[gez]

Actually just figured out these are basically the same thing. What value resistors would I need or how do I calculate it if I am looking to get the transistor into what the saturated region?

Use a Si transistor, check its data sheet for max base current allowed, then select your base resistor to keep well within this limit (don't push it).  If loading on the low-voltage logic output is an issue, use a high gain transistor and make the base resistor larger.  As far as the collector resistor goes, the larger it is the more chance of pushing the transistor into saturation.  Don't forget, however, that this resistor pulls up whatever follows...might be an issue if it forms a divider with some resistance to earth.

[Gus]

What do you need?  First you need to define what you want to drive and what you need.  You do not have to post anything but the logic chip number or output circuit type of the chip and input circuit fragment that needs a 6V to 7 VDC(what ever you mean by that)

Are you posting 2 to 3 meaning 2V to 3V  change(a  change of 1V) or 0V to about 2V or 3V?

same for the 6 to 7 part change of 1V or 0V to 6V to 7V?

Or is it a gain stage to amplified 6/2 for a X3 gain stage?

One answer could be a level shifter and the other an open collector type circuit or you could even use an opto like a 4nxx type and the other a gain stage

[gtrplaya101]

Ok some cleared up details. I am building a rack controller and am having some issues. I have three MM74C373N chips that drive 5v relays. The output from the chip is 2v exactly. I just need the output to be above 5 volts because I am using a 5v regulator to control the voltage to the relays. Since there are 12 of these outputs that need to be amplified I was thinking a transistor would be the easiest way to do so.

[waky]

if the chips output voltage isn't stable i THINK you could use a schmitt trigger to get 2 precise voltages from the semi varying one and then you would use the inverting buffer someone said earlier to get your 2-3 volts

[gtrplaya101]

No the output IS stable. Earlier I wasn't sure what it was so I was making an approximation. BY the way thanks for the help so FAR!!!

[gez]

I just need the output to be above 5 volts because I am using a 5v regulator to control the voltage to the relays.

So you're switching the 5V regulator on and off to control the relays?  If that's the case, what I outlined won't work and what you've outlined seems a little odd.

[gtrplaya101]

Yes I know is a little odd but I wanted to be able to use the controller at several different voltages and the regulators gives me that flexibility.
So can I not use a transistor as a switch to pass a higher voltage when a lower one is detected at the base?