TLV9004: TLV9004 : Buffer and Voletage Divider

Hi Si Yoon Kim,

here is the phase stability analysis for the compensator circuit:

si_comp_phase.TSC

Kai

Dear team,

Your compensator circuit answer was very helpful.

I'm really sorry, but can I ask for answers about Question 1),.2).?

I am very appreciate for your response. Thank you

Hi Si Yoon Kim,

for me this 4.53k resistor is nonsense. I see no reason to install it. It's only effect is to erode the phase margin and cause instability.

Furthermore, as the TLV9004 is a CMOS OPAmp with ultra low input bias currents, there's absolutely no reason to provide an input bias current cancellation. 5pA x 4.53k = 23nV. So, the input offset voltage of 0.4mV is 17400 times bigger...

Kai

Hi Si Yoon,

I will address questions 1 and 2 here.

The phase margin will not affect the output value significantly unless the circuit is unstable or approaching instability.  Otherwise the change in phase margin caused by a resistor in the feedback loop is only an output overshoot during a transient response.  If the circuit has plenty of phase margin, then this really does not matter.

As for tuning the circuit via this resistor, the bias current of the amplifier is too small for this to make sense, as Kai has pointed out.  Such small changes in the output voltage due to the bias current will be dominated by the offset voltage.

If you do not know the purpose of the R3 resistor, can it simply be taken out?

Regards,

Daniel

Dear Team

Thank you for your reply. Your answers are very helpful for me.

1). Then, for example, to replace OP-Amp with "LMH6643",
Power supply based on 5V supply, input bias current = -1.7uA (typ),
If R = 4.53k, Vos = ± 7mV (max), can it be viewed for the purpose of removing the input bias current?

lmh6643.pdf

I am very appreciate for your response. Thank you.

Hi Si Yoon,

Regarding question 3, Kai's method is correct.  This is the proper way to simulate phase margin.  You can learn more about it in this presentation.  Note that the purpose of C1 is to model the input impedance of the amplifier and that "Vfb" in his circuit is the node of primary interest as it is equivalent to Aol*Beta.

For your circuit, you can also perform a slightly more simple analysis to get the phase margin.  It is similar to the method Kai has shown, but you do not need to include the input capacitance of the part as an extra circuit capacitor.  Here is an explanation of the method.

Regarding questions 4 and 5, before you run a stability analysis on the integrator, you need to ensure there is a proper DC biasing of the circuit.  In other words, you need to make sure that the amplifier is in its linear region of operation and not at one of the two supply rails.  To do this, simply take the circuit and before you run your stability simulation run a DC simulation and check the voltage at the output of the amplifier.  If it is not too close to one of the supply rails, then your circuit has found a biasing point for stability simulation.  If it is too close to a rail, then adjust the DC voltages or circuit component values.

Regards,

Daniel

Hi Si Yoon Kim,

the LMH6643 is a HF OPAmp. This is a total different story.

Kai

Dear Team

Daniel and Kai and TI Team, thank you for your detailed and kindly replies. I learned a lot because of your explanations.
I wonder One thing, I asked "LMH6643" as an example.
If it is an HF OPAMP, is it a High Frequency OPAMP?
Can you tell me what HF OPAMP is in detail?
What is different from the existing CMOS AMP?
And, if i replace OP-AMP with "LMH6643" in case1-2, is there a big problem?
i'm curious about this.

Thank you very much for your answer.