| Electronics > Projects, Designs, and Technical Stuff |
| fully-differential amplifier with discrete devices |
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| Zeyneb:
Hi Tim, --- Quote from: T3sl4co1l on May 02, 2019, 01:14:25 am ---To do that, you need to use two BJTs in parallel for the tail current, hence setting twice the collector current so it balances normally. --- End quote --- Have you seen my two Widlar resistors R2 and R3. I thought I already dealt with that in my design, or am I missing something? --- Quote from: T3sl4co1l on May 02, 2019, 01:14:25 am ---But you cannot avoid the use of the Vocm error amp, even with perfectly matched SPICE BJTs I think you will find :) --- End quote --- Yeah, But like the FDA by TI I can apply it as shown on the 2nd stage and that will also deal with the voltage levels on the collectors of Q1 and Q2, right? --- Quote ---it's called a folded cascode --- End quote --- Lets see what I can find on folded cascode on the internet! Thanks, Zeyneb |
| T3sl4co1l:
Yes; I forgot to finish my thought. The emitter resistors adjust the current, but again it will never quite match. To control the current, you could put an emitter follower in series with the resistor, like used here: https://www.seventransistorlabs.com/tmoranwms/Circuits_2008/Triangle.gif but that's where I left off because you still have a degenerate case of two CCSs fighting. The folded cascode works on the sums and differences of currents; it isn't a fight because the summing node is low impedance, not a high impedance gain node. :) Tim |
| magic:
A similar circuit invented by one audiophile vendor does away with the Vocm servo by weakly pulling emitters/sources of the diff pair to ground, see US patent 5,376,899 which by the way TI was forced to license ;) |
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