Current booster questions (updating components)

[randomOracle]

Hi everyone,

A while back I found the attached circuit and I can't figure out how to redesign/adapt the circuit. The components being used are relatively old and I 'just' wanted to replace unavailable components with newer ones, otherwise looking for a similar performance. I do not fully understand how the circuit works, but it seems to me that R1 and R2 'somehow' are selected such that the transistors are off when the OPAMP is not pulling any current -- they only switch on once the OPAMP actually starts working. All the calculations I came up with lead to inconclusive results though.

Perhaps someone is able to explain to me how R1 and R2 were computed for the specific example, such that adapting the values for other opamps, and especially transistors, is possible?

Thank you.

[Terry Bites]

The LT1210 is a monster.

We've been here before..
https://www.eevblog.com/forum/beginners/non-standard-op-amp-configuration/
https://www.eevblog.com/forum/projects/push-pull-output-stage-1m$-question/

When the opamp current its output current is greater than about Vbe/6.2 the power transistors turn on supplying more current into the load. A short circuit limit set by the emitter resistors and the flames around them.
WRT to current limiting this circuit is poor.
The quiescent current of the opamp has little effect. Just choose your to create a Vbe at a point below the current limit of the opamp and off you go. If you use Darlington's you'll need to generate 2xVbe. Dont try this circuit with MOSFETS!!

[randomOracle]

so just to confirm this: so Vbe is around 1.0-1.2V (depending on current) according to the datasheet, so 1V / 6.2R = 160 mA; thank you!
most other transistors I checked would work well with the 6.2R, too; nice.

[David Hess]

The D44VH4 and D45VH4 are part of an old series of ring emitter transistors which are still produced in the form of the D44 and D45 series and replaced by the D44VH10/D45VH10 or D44H11/D45H11 if slightly lower power is acceptable.  These transistors would still be my first choice unless lower power is acceptable.

[David Hess]

so just to confirm this: so Vbe is around 1.0-1.2V (depending on current) according to the datasheet, so 1V / 6.2R = 160 mA; thank you!
most other transistors I checked would work well with the 6.2R, too; nice.

That is the saturated Vbe.  In practice it will be more like 0.8 volts but the values are not critical for proper operation.