[SOLVED] Circuit solution for OPA355

Circuit solution..............................

I got a problem given in my class ,trying to figure out the problem.

1. Design a two stage transimpedance amplifier using the picture

solution:
Since its transimpedance ( current to voltage conversion).So i have to design two transimpedance amps which has total gain.What value of the resistors i have to take for input terminals (Rin) to Rf


this is my design ,need help to figure out the values and modification

kdg007 said:

So i have to design two transimpedance amps which has total gain of 6M.

Click to expand...

No. You have to design one transimpedance amp, using two chips.

The obvious approach is to configure the first stage as a transimpedance amp, and the second stage as an inverting voltage gain stage.

The circuit below has the right gain, but the speed is lousy. You need to figure out how to change the resistors to get the speed you want. You might also want to add capacitors to shape the frequency response.

1)do u mean i need to keep capacitors in the picture above >?
2)if i keep the input to +ve terminal instead of -ve terminal for the first stage,i wont be needing the second stage for voltage inversion right ?
3)i didnt get the gain of 6M ..maybe my calculation was wrong

kdg007 said:

1)do u mean i need to keep capacitors in the picture above >?

Click to expand...

Maybe you will need one of those or both of those. Maybe not. If you want the frequency response to roll off below 100KHz, you need to add another capacitor.

2)if i keep the input to +ve terminal instead of -ve terminal for the first stage,i wont be needing the second stage for voltage inversion right ?

Click to expand...

How can you make a transimpedance gain stage with the input connected to the +ve terminal? Where will you connect the feedback resistor?

3)i didnt get the gain of 6M ..maybe my calculation was wrong

Click to expand...

The gain of your circuit is not 6M. The gain of the whole amp = gain of first stage multiplied by gain of second stage.

i got the understanding of the circuit.thank you
1) Rise time: 400nsec worst case : i have to adjust the pullup resistor at get the desired Tr right ?
2).how do we select the value of the capacitor ?

First things first...
Here's the opa355 datasheet in case you haven't already got it: https://www.ti.com/lit/ds/symlink/opa355.pdf.

• On page 3 it says the output can source or sink about 60mA, so you don't need a pull-up resistor.
• Make life easy: If you use a split supply of +-2.5V, instead of a single 5V supply, you don't have to worry about biasing.

Now a bit of theory...
For any simple first-order RC filter (high pass or low pass), the -3dB corner frequency is given by: F = 1 / (2*Pi*R*C). See here: https://en.wikipedia.org/wiki/RC_circuit#Frequency-domain_considerations

In the circuit below, R1 and C1 form a low pass filter, R2 and C2 form a high pass filter, and R3 and C3 form another low pass filter.

A bit of confusion...
The question says "Frequency response: 100KHz to 500KHz". It's not clear if frequency response has to be at least that wide, or if it must be limited to that range.

Actually, it's a trick question. Rise time is linked to frequency response. See here: https://en.wikipedia.org/wiki/Rise_time#One_stage_low_pass_RC_network. For a simple RC low pass filter, rise time = 0.35 / bandwidth, so if you roll off the frequency response at 500KHz, the rise time will be 700nS. Oops! It's very clear that rise time must be lower than 400nS, so frequency response must extend to at least 875KHz, preferably much higher for faster rise time.

IMHO:

• Leave out C3. You don't need it, and it will slow down the circuit.
• You probably do need C1 for stability, but make it small enough that it doesn't hurt the frequency response too badly.
• You definitely need C2. Without it, DC offset will be really bad (but your simulation won't tell you that). Make it big enough that the R2C2 corner frequency is well below 100KHz.

and some practical issues...
Even without C1 or C3, the frequency response will be limited by the opamps. There's a nice graph on page three of the datasheet, showing the frequency responses of inverting gain stages with different gains. For a gain of -10, the response rolls off above 15MHz, so for a gain of -100, the response will roll off above 1.5MHz.

That might be a good first guess for the gain of the second stage of your amp. If you make the gain lower, it will be faster but you will have to use a larger value for R1, which will slow down the first stage.

You need to play with the simulator to find what combination of 2'nd stage gain and R1 resistance gives the best overall speed.

One more thing.
If the resistance if R2 is too high, noise gets worse and the speed and stability of the second stage start to suffer.

OTOH, if R2 is too low, the first stage will struggle to drive it. IIRC from the datasheet, the opa355 is happy driving about 500 Ohms, so I'd set R2 a bit higher, maybe up to 1 or 2K. IMHO, 10K is too high.

thank you for making things simple to understand took the confusion out of my mind now..