• Resolved

OPA2333: Does the follower feedback loop need resistance?

Prodigy 170 points

Replies: 1

Views: 47

Part Number: OPA2333

1. The following circuit of opa2333 is shown in the figure. Is R7 required? Why?Are other operational amplifiers needed?

2. Can internal clamping diodes be used? In case of over-voltage, the clamping diode current is 0.7ma, less than 1mA in the manual. Is it OK?

3.Does the output of the op amp need a resistor in parallel to ground to ensure that the op amp has some load? If R12 in the figure is needed, how big is the resistor?

  • Please see my answers below:

    1. The following circuit of opa2333 is shown in the figure. Is R7 required? Why?Are other operational amplifiers needed?

    In short, it depends. Since OPA2333 includes back-to-back parasitic diodes between the input terminals, the resistor in the feedback may be used to limit the current through the diodes to a rated magnitude of +/-1mA or less.  Since the circuit above already includes R4 of 4.7k, I do not believe you need R7 unless you need to roll-off the signal below 300kHz by adding a feedback cap in parallel with R7.

    2. Can internal clamping diodes be used? In case of over-voltage, the clamping diode current is 0.7ma, less than 1mA in the manual. Is it OK?

    Yes, the internal diodes may be used to carry current of up to +/-1mA or less.

    3.Does the output of the op amp need a resistor in parallel to ground to ensure that the op amp has some load? If R12 in the figure is needed, how big is the resistor?

    No.  There is no need for load resistor.  However, with C19 10nF capacitive load your circuit has only 2 degree phase margin - see below - and for all practical purposes the output will oscillate.

    .

    You may confirm marginal stability by running a small-signal transient simulation - see below excessive ringing/overshoot.

    In order to meet recommended minimum phase margin of 45 degrees, a series output resistor must be increased to 400ohm - see below.

    You may confirm the stability of the circuit by running transient simulation - see below.

    For your convenience, I have attached AC and transient simulation schematics.

    OPA333 AC Stability.TSCOPA333 Transient Stability.TSC

    Marek Lis, MGTS
    Sr Application Engineer
    Precision Analog - TI Tucson