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I'm trying to design a power amplifier for the HF band to drive an inductive charger. The stage I'm working on now is a voltage amplifier which needs to be able to amplify 1 Vpp to 10 Vpp.

The input signal (1 Vpp) can vary from 5-15 MHz. The output signal will drive the MOSFET gates of the second stage, so it doesn't need to be able to provide a lot of current.

The question is what kind of circuit topology should I be looking at to solve this problem? Some configuration of BJTs, mosfets, op-amps? Also, what parameters do I need to look for in choosing the proper components?

It would be great if it was non-inverting, with little or no phase offset, but that is definitely not a necessity.

My First Try: I tried to build the circuit from this page, which says that a common-base amplifier is well suited for high voltage gain, and high frequencies. I chose this BJT to build it since it's specified as an RF transistor. Unfortunately, I can only get a little bit of gain around 10 MHz with the right biasing, but not with a 1V input, not anywhere near 10X, and it only works really close to 10 MHz.

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  • \$\begingroup\$ Unless those mosfets are TINY you are going to need more current then you think... The miller capacitance will have you in a big way, and I would not be surprised to see |Z| of the mosfet gates in the order of a few tens of ohms depending on exactly what you are using. \$\endgroup\$
    – Dan Mills
    Mar 26, 2017 at 1:45

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An opamp with GBWP (gain bandwidth product) of 200MHz will probably do the job and is easily configured as a non inverting gain of ten amp. One word of warning though... Driving a mosfet can be tricky because of gate capacitance and you might need to connect the opamp output via 100 ohms. All this assumes sinwave amplification. If wanting to amplify square waves a quite different solution will be required.

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  • \$\begingroup\$ In addition to the GBP is the slew rate a factor? Do you have any op amps which you can suggest for this situation? I assume square waves are more difficult because of the higher order frequencies present in the spectrum. Thanks for your input! \$\endgroup\$
    – Keegan Jay
    Oct 25, 2014 at 7:33
  • \$\begingroup\$ slew rate is a factor but you tend to find slew and GBP run hand in hand. No recommendations for opamp other than look at analog devices site and use their tables. Linear tech and ti also have good search engines. Just about to board a plane so tricky to look for you. \$\endgroup\$
    – Andy aka
    Oct 25, 2014 at 7:56
  • \$\begingroup\$ @JayKeegan Maybe an LM7171 ti.com/lit/ds/symlink/lm7171.pdf.. It's hard to find speed and bigger than 5-6 V supply. Lots of other high speed stuff is CFA. (And also lower voltage.) \$\endgroup\$
    – George Herold
    Jan 23, 2015 at 20:32
  • \$\begingroup\$ Indeed the LM7171 is the fastest VFA I could find. \$\endgroup\$
    – Keegan Jay
    Jan 25, 2015 at 6:09
  • \$\begingroup\$ Just to give one more option: The LT1227 "Current Feedback Amp" would also be more than fast enough. Even comes in DIP package for easy prototyping. \$\endgroup\$
    – Nils Pipenbrinck
    Mar 29, 2015 at 10:12

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