THS4042: Can THS4042 drive 0.1uF output capacitance with Figure 68. Noninverting, Unity Gain Schematic on page 17?

Hello Kazuya,

1) Yes, you can use the unity gain configuration shown in figure 68.

2) Trying to drive a large capacitive load, you will want to add an isolation resistor (20 ohm in figure 65) in order to maintain proper phase margin. This resistor doesn't necessarily need to be 20 ohms but should be tuned to keep the amplifier stable. The issue with adding isolation resistor is this will create an RC lowpass filter that will lower the bandwidth of your circuit. I would recommend simulating to determine what value would work best for you.

3) Are there any other conditions you are looking for? What frequencies are you operating at? Typically amplifiers with higher output current capabilities will be better at driving capacitive loads.

Best,

Hasan Babiker

 Hello Hasan,

 Thank you very much for your reply.

 Regarding to 3), The customer needs a ope-amp which can be used for higher frequency. They want to input 500kHz signal.

Acording to Figure 6 on page 6 of the device datasheet. the amp gain is getting higher than 1 at 500kHz.

 Do you have any device which can keep gain 1 with 0.1uF driving and 500kHz signal input?

 Thank you again and best regards,

 Kazuya.

 Hello Hasan,

 The customer wants to input 500kHz sine curve with 10Vp-p.

 Could you please tell me any device can be used for the customer?

 Thank you and best regards,

 Kazuya.

Hello Kazuya,

On the precision amplifier portfolio, (amplifiers with <~50MHz bandwidth), the OPA211 with a 3.18Ohm isolation resistor offers a f(-3dB) corner frequency of ~508-kHz, and a phase margin of ~45.9 degrees (the amplifier is stable). 

However, I should mention the total harmonic distortion for the OPA211 is only specified to a frequency 20-kHz. The OPA1611 is an audio amplifer device that could work as well, however, distortion for the OPA1611 is only specified up to 100-kHz.

Since your application requires a 500-kHz input signal, there may be other High Speed amplifiers options that may work well on the 500kHz frequency range providing low distortion.  For example, the THS4031/2 is a high-speed amplifier with 100-MHz bandwidth amplifier that specifies THD performance at higher frequencies that may fit this application (using proper compensation for stability).  I will refer to the High Speed amplifier applications team for other high speed amplifier suggestions that may fit the 500-kHz input frequency requirement.

Thank you and Regards,

Luis

Hello Kazuya,

Have you considered using a current feedback amplifier? The amplifiers typically provide higher output current and slew rates, making them especially suitable for driving large capactivie loads. Some suggestions you should look at are the THS3121 and the THS3111. 

Unfortunately for other high speed amplifiers that are voltage feedback (such as the THS4032 & THS4042), the isolation resistor you will need to use to stabilize your circuit will be too large and bring you bandwidth below 500kHz. 

Best,

Hasan Babiker

Hello Kazuya,

Your 10Vpp, 500kHz requirement is actually a modest slew rate requirement (assuming a sine wave for now?). 

However, if you are really driving a 0.1uF Cload (before we even think about stability issues) that 5Vp*2pi*500kHz = 15.7V/usec slew rate will be asking for this peak current 

Ipeak = 0.1uF*15.7V/usec = 1.57Amps!!

Need to constrain you selections to those devices that can do that, and dissipate the power. 

Hi Kazuya,

One possibility may be the OPA564, able to drive +/-1.5A continuous, and may be able to support the 5Vp signal at 500-kHz driving the capacitive load.  I will look into this detail tomorrow.

Thank you and Regards,

Luis

 Hi Luis,

 Thank you very much for the information.

 I'm looking forward to receiving your reply.

 Thank you again and best regards,

 Kazuya Nakai.

Yes, the OPA564 looks very promising, starting from the reference design, the attached looks promising, 

This is using the imbedded integrator approach to driving a cap load (that 1.1ohm output with the 110pF inside the loop Cfeedback, 

Set this up for gain of +2V with +/-10V supplies, AC sim looks promising, I was targeting 1MHz, F-3dB, little bit peaked, I had adjusted the current limit R to target 1.7A

Then the 500kHz +/-5V output looks a little peaked (+/-5.2V) which can be adjusted, 

Here is the TINA V9 file, good luck, 

OPA564 Cload driver.TSC

Hi Kazuya,

The OPA564 supports a +/-12V supply maximum.

Thank you and Regards,

Luis