Hi Voltage Discrete Opamp Application

[nerdman]

An application for the DIY community I thought would be fun to share. Has some application for tube amps as well. If the verbage sounds a bit commercial or if the post violates the commercial rules I apologize. Not my intention. Its from an app note i wrote in October 2011.

A 990Enh-Ticha discrete opamp provides a low noise front end for this amplifier, which is capable of delivering over ±300mA to a load with a 90V peak-to-peak output swing. Transistors Q1 and Q2 are series regulators stepping down the supply voltage for the 990Enh-Ticha opamp to approximately ±24V, while transistors Q3 and Q4 provide the high current output drive. R3 and R4 form an output voltage gain stage whose gain, Av=3, is reduced to unity at high frequencies by C1 to maintain stability. The overall gain is set by R1 and R2. In the circuit example shown in PDF, the overall gain is 26dB (Av=20) with C2 setting the upper bandwidth 3dB point at 100kHz.

Q1 and Q2 should be capable of a continous current of 100mA and a Vce of at least 50V. For a margin of safety, attention should be paid to the transistors’ safe area operating regions. Q3 and Q4 are high current power transistors that will require heatsinks depending on load resistance. Several part types are called out in the schematic.

Power supply decoupling capacitors are not shown in the circuit diagram for operational clarity. 22uF low ESR electrolytic capacitors on the supply rails to ground are highly recommended. The 990Enh-Ticha opamp has 0.1uF capacitors at its VCC and VEE pins internally. Adding a small capacitor (100pF-1nF) across R6 and R7 can be helpful if the supply rails have ripple on them.

[beun]

Don't want to burst your bubble, but this is an old technique. Also the bias current of the output transistors is not going to be accurate and will probably vary wildly with temperature.

[nerdman]

my bubble is not burst. we can clearly add the cc sources and mirrors to lock down bias and compensate for temperature. it was originally intended to simply show an idea on how to run an opamp on a hv supply. i appreciate your pointing this out and i am interested in seeing how the online hive mind contributes to solving different engineering problems. My other post has similar short comings and again I hope to see how things unwind. be excellent!

[beun]

The obvious way is something like this. The resistor in the emitter stabilizes the bias current and the capacitor keeps the signal gain constant.

[gootee]

There are some interesting high-voltage-output and current-boosted opamp circuits in AN18 at linear.com, and, in Figure 14 (which looks a lot like the OP's circuit) and Figure 33 of AN106 at linear.com, and, in AN-272 at national.com, where I particularly like the one with the McIntosh 75 tube amp inside the opamp's feedback loop.

[qusp]

Originally Posted by gootee

in AN-272 at national.com, where I particularly like the one with the McIntosh 75 tube amp inside the opamp's feedback loop.

haha, do you think they were having a laugh?

[davygrvy]

An alternate name for this technique is High Dynamics OpAmp or HDOA