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Current Amplification using BUF634 high speed buffer

Sailesh Vempaty
Prodigy 165 points
Other Parts Discussed in Thread: BUF634, THS3091, LM7171

I am looking to amplify the current of a 3.3v, 50% duty cycle, 40MHz square wave from the output of my function generator. I need this signal to drive a 50 ohm load with a drive current requirement of approximately 65mA. The output impedance of the function generator is 50 ohm and the output square wave signal is being fed through a 50 ohm BNC cable. I am thinking about using an Op-amp and Buffer combination to amplify the square wave signal from the function generator in order to provide the required drive current. I want to use the BUF634 high speed buffer device in a closed loop with an op-amp to achieve my requirements. I would think a gain of 2 or 3 would be sufficient.  

I am somewhat unclear about how to design this circuit. Could a suitable op-amp be suggested to compliment the BUF634 device (Needs to be capable of driving high speed signals e.g. 40MHz) ? I also want to know what supply voltages are required to be supplied to both the V+ and V- rails of the BUF634 and the op-amp for my requirement. It would really help to have a schematic to visualize what type of circuit I am dealing with.

  • 0
    TI__Genius 12631 points
    Sailesh,
    You do not really need to use a the buffer configuration shown. You can simply use a fast, high output current drive opamp like the THS3091 in a non-inverting configuration with a gain of 2V/V (6dB) to achieve this.
    Can you be clearer on the load you are driving? Is it a resistor or some other kind of transducer?

    -Samir
  • 0 in reply to Samir Cherian
    Prodigy 165 points

    Hi Samir,

    Thank you for responding. I was also wondering about this and I would like to try this with the following op-amp as it comes in a DIP package: www.farnell.com/.../1902708.pdf

    I think this will suit my needs as it can drive an output current of 100mA?

    The load I am driving is actually a laser with a 50 ohm input impedance. It requires approximately 65mA drive current to operate. 

    1) I am thinking of having a configuration like the one I have attached in this comment. Could you tell me if this would work? 

    2) The 3.3V square wave  (0 to 3.3V) will be fed through a 50ohm BNC capable to the input of the op-amp. The output from the op-amp will also be fed through a 50ohm BNC capable to input of the Laser. Both of these cables will be approximately 1 meter long. I read a little bit about impedance matching somewhere and hence I have a 50ohm resistor (R3) shunted to ground at the input of the op-amp and a 50ohm resistor (R4) in series with the load (laser - 50ohm input impedance) at the output of the op-amp. Could you tell me if this is in fact necessary and whether I have done this correctly? 

    3) My other question is if I in fact need this 50ohm resistor (R4) at the output in series with the load then will I need a unity gain buffer before the 50ohm series resistor so that the impedance effect of this resistor does not impact the overall impedance of the negative feedback loop (R1 and R2)?

  • 0 in reply to Sailesh Vempaty
    TI__Genius 12631 points

    Sailesh,

    You can certainly use the LM7171 for your application.

    With the configuration you have shown there will be a voltage drop across the 50Ohm resistor. You need to look carefully at the laser diode datasheet for the VI curve. I am not sure you can really model the laser diode as a fixed resistor since it behaves more like a diode. Once you cross a certain threshold value the current across it will rapidly increase with only a small change in voltage. Imaging if you have 3.3V input times a gain of (1+600/200) = 4*3.3 = 13.2V. That means the voltage across the laser diode will be half that at 6.6V.....are you sure your laser diode can take that? In my experience you really need a V-to-I converter circuit like the ones shown in here.

    -Samir

  • 0 in reply to Samir Cherian
    Prodigy 165 points
    Yeah Samir I think my first approach will be to try the circuit at a gain of 2 and remove the 50ohm resistors at the output and input. That will give me 3.3*2=6.6V and at a 50% duty cycle that will be 3.3V across the laser. My only worry is if there will be a impedance mismatch due to removing the 50ohm resistors since the input square wave arrives at the input of the non-inverting configuration via a 50 BNC cable and the output square wave arrives at the laser input via a 50ohm BNC cable too.

    I found out that the laser requires 3.3V for digital modulation. For your reference the datasheet for the OBIS 375 lx laser is below:
    www.coherent.com/.../COHR_OBIS_Family_DS_1115revI_1.pdf

    I cannot open the link for the V-to-I converter circuits by the way.
  • 0 in reply to Sailesh Vempaty
    TI__Genius 12631 points

    Sharad, please try this link

    or google SBEA001