What’s the minimum requirement to use opamp as the input stage of the power amplifier? It might be:
1, Good voltage swing, as the power rail voltage is often limited.
2, A bias network that could provide a fixed voltage for the output stage.
Here we go.
The bias network is bootstrapped, so you loose little to none voltage swing. The output stage could be mosfet or EF2, etc. Change the resistors from the bias network to fit your need.
1, Good voltage swing, as the power rail voltage is often limited.
2, A bias network that could provide a fixed voltage for the output stage.
Here we go.
The bias network is bootstrapped, so you loose little to none voltage swing. The output stage could be mosfet or EF2, etc. Change the resistors from the bias network to fit your need.
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The limitation is the opamps voltage supply.
Most opamps has max +/-20 volt supply. There are however opamps with more supply.
But yes, this is a nice way to make a poweramp you have designed.
Most opamps has max +/-20 volt supply. There are however opamps with more supply.
But yes, this is a nice way to make a poweramp you have designed.
Yep.
I thought putting another gain stage in front of the output stage. It would essentially creates a VAS. The design would complicate very quickly if I go that route.
1. If the (not an opamp) parts of the circuit contribute any voltage gain, the frequency compensation scheme needs to take this into account.
2. If the (not an opamp) parts of the circuit contribute any significant phase shift (say, > 10 degrees) in the neighborhood of the gain crossover frequency, then the frequency compensation scheme needs to take this into account.
There's a reason why opamp-current-booster ICs like the LT1010 and the BUF634A and the HA5002 (my favorite), provide zoomed-in and detailed graphs of phase shift vs frequency, between 100 kHz and 30 MHz. The manufacturers are keenly aware that composite amplifier stability depends on both the upstream opamp and also the downstream current booster.
2. If the (not an opamp) parts of the circuit contribute any significant phase shift (say, > 10 degrees) in the neighborhood of the gain crossover frequency, then the frequency compensation scheme needs to take this into account.
There's a reason why opamp-current-booster ICs like the LT1010 and the BUF634A and the HA5002 (my favorite), provide zoomed-in and detailed graphs of phase shift vs frequency, between 100 kHz and 30 MHz. The manufacturers are keenly aware that composite amplifier stability depends on both the upstream opamp and also the downstream current booster.
Have you tested the GainMargin and Phase Margin?
You may need a capacitor from opamp output to the inverted input.
You may need a capacitor from opamp output to the inverted input.
As long as keep the unit loop gain bandwidth below 1MHz, it should be fine. Take OPA2134 for example, from its manual, it has “Gain bandwidth product” 8MHz. If the close loop gain is 20 times. Thus, the unit loop gain bandwidth is only 400KHz. I don’t need small cap on the feedback resistor.
Most output devices can do 1MHz. It might be a stretch for some really slow devices such as 2n3055. Lower the bandwidth accordingly if you choose slow devices.
Check with at least 20kHz sine wave signal rail-to-rail to see if the circuit is able to provide enough current to the mosfet gate input, since the input capacitance is considerable (Ciss=1700pF).
For source follower, the input capacitance is essentially bootstrapped by the output. That should not be an concern. Will do 100K square wave to demonstrate that.
Sure, for digital source 20KHz sine wave and Vpeak=15V, you only need 1.8V/us.
Even if we consider an input signal filtered @100KHz (LPF) you need 9.4V/us.
Interesting topology!
Even if we consider an input signal filtered @100KHz (LPF) you need 9.4V/us.
Interesting topology!