Help me decide if this opamp circuit will work as purposed

Question

please help me decide if this will work:

Primary objective: Amplify 0~3v input signal to 0~12V output signal.

I understand that this whole setup is a bit unusual, since I can simply connect the +input of opamp to 12VDC, and with my resistors' values, 3v input could be amplified to 12V (I'm 80% sure). But nevertheless I want to know if this odd scheme will work? I think I'll have to be very careful with my OpAmp output, which dictates the 3904?

Edit: My most sincere apologies there SHOULD BE a pullup resister between the collector and the 12V, for some reason this slipped through my double check, incredible, I'm new to eagle and the copy paste in it isn't that intuitive.

The resistor has a value of 2K.

Edit #2:

Sorry for the delay but a few things to clarify, apologize again if I overlooked your question, there are too many and I'm kind of really busy:

1. 200mA is supposed to be the current drawing in. This whole setup is kind of to sink current from lower-tier components that will be connected to this interface, so 12v/2K = 6ma isn't what I'm talking about.

2. The resistor should be 90k/30k, as some of yous suggested, my mistake.

3. Oscillation: I believe your concerns are quite legit but please be advised that the input will be mostly "static" voltage signals, meaning it rarely changes more frequently than 5 times per second, so I'm skeptical that it will be that huge of a problem, I could very well be very, very wrong.

   • Matt

Answer 1

No, your circuit will not work. The main problem is that 12V is connected directly to Vout. And Q4 is going to short 12V to ground (which will destroy Q4).

Look at this schematic. Just for inspiration.

^{simulate this circuit – Schematic created using CircuitLab}

UPDATE:
I changed value of R2 from 120kΩ to 90kΩ. It is wrong in question too. If OP wants to amplify from 0-3V to 0-12V than there should be 1:3 ratio for R1:R2

UPDATE2 - EXPLANATION:
This circuit makes Vout = 4* Vin

If you increase Vin, output of U3 will go lower, which will make mosfet M1 more open (Vgs is more negative = Rds is lower) and thus Vout increases. It will increase until voltage at non-inverting input of U3 is equal to inverting input (which is Vin).

If you increase Vin, output of U3 will go higher, which will make mosfet M1 less open (Vgs is less negative = Rds increases) and thus Vout decreases. It will decrease until voltage at non-inverting input of U3 is equal to inverting input (which is Vin).

Answer 2

Your gain is 1+ since you have inverted the feedback the Vin- is now the positive input so Av=1+R2/R1 assuming you use something 1k Pullup and you REALLY NEED the transistor to pull down a charged cap with high current and small pullup current.

This a poor inverting negative peak detector with storage cap and missing pullup resistor with a gain of +5?

Where are the rest of your design specs?

Answer 3

It's a good start, but I see two flaws.

1: your output terminal is connected directly to 12VDC, you probably forgot to draw a resistor between the 12VDC and the transistor. given your choice of 2K for the base resistor, 1K seems about right in this place.

2: C2 between output and ground seems likely to cause trouble (amplifier oscillation). you probably don't want it. or perhaps you want it between 12VDC and ground.