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Precision Amplifiers Forum
I am trying to amplify a 2 Vpp 10 MHz signal with the OPA564 power amplifier wired as a non-inverting amplifier. I am supplying +12 Vdc to V+PWR and V+; V- and V-PWR are to ground; and Vdig is at 3.3 V. E/Sbar is floating (to keep it permanently on). For some reason I get only noise on the output pins. Can anyone suggest what might be going wrong and how to fix it? Thanks for the help.
Do you have the current limit resistor also connected to ground? If this resistor is not in place the OPA564 will not work correctly.
Yup, Sorry, I didn't mention that I have a 15 kOhm resistor connected to Iset, which should give me a output current limited to 1.2 A.
Just to confirm, I don't need two different supplies for the OpAmp and the power output stage, correct? They both run off of a single supply +12 V?
Also to confirm, if I supply Vdig and then Vs one after the other; for example by using two mechanical switches, this will satisfy the power supply sequencing requirements, correct?
I think your power sequencing is fine. If you are not getting an output signal in a simple non-inverting configuration there is something fundamentally wrong that is going on here. You should have no problem even when using a single +12V supply. Have you verified that you can first get a correct DC signal at the output (unloaded) before trying to input your 10Mhz input? Is your common mode signal with the range of V- to V(+) - 3V? Is the output saturated into either the ground rail or positive supply?
Thanks for the help. I used a new chip and it works under the specs I gave above. I must have done something to the previous chip. I have a new problem with the chip though. I need to amplify a 10 MHz signal coming from a crystal oscillator to 1.5A and 5-10 V. The chip is capable of this based on all of its specifications in the data sheet. However, when I supply a 1 Vpp input signal, I get a ~0.94 Vpp output (haven't measured current). My non-inverting op-amp has a feedback resistor of 4.7 kOhms, and a 1 kOhm resistor to ground. So i should be getting a gain of ~5.7.
Do you have any idea of how I could get a system that can amplify the crystal oscillator signal to the 1.5A and 5-10 Vpp spec? (Maybe another configuration with the OPA564, or another chip that you are aware of that can do this, or a completely different circuit that I could try)
You are up against a couple of things here: (1) Thermal Power Limits on the OPA564 package (2) Output Swing vs. Load Current (3) Full Power Bandwidth of the Amplifier. If you boosted the power using an external transistor output stage and reduced the drive power of the OPA564 it might be possible to do it this way. You will probably need at least a +15V supply to give yourself enough headroom to achieve +10V out.
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