Found a mention about
White Emitter-Follower on a time-nuts mailing list, while researching about DCF-77 receivers. Didn't know the term, searched and found it doesn't have to do with time keeping.
White emitter-follower is a circuit that behaves similar with the single transistor emitter-follower (common collector), but with many of the pitfalls of common collector fixed by a second transistor added and some feedback.
- it's an improved emitter follower, patented by Eric L. C. White in 1944, for tubes, AKA White Cathode Follower (WCF)
- advantages in comparison with emitter-follower
- symmetric response
- better slew rate
- amplification closer to 1.0
- higher input impedance (about 30 times)
- much smaller output impedance (about 100 times)
Patent, origins and circuit analysis:
https://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_04/Sec_19/991_White_Follower_Optimization.pdfhttps://trace.tennessee.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=3833&context=utk_gradthesSimulated schematic is from the thesis in the second link.
Notes:
- normal emitter-follower
- asymmetric push/pull current capabilities (asymmetric raise/fall slew-rate)
- asymmetric amplification for each alternance (transistor's beta is bigger at bigger Ic)
- output impedance in the range of few ohms
- voltage gain almost 1, but worst than the White emitter-follower
- White emitter-follower
- might oscillate for Rgenerator 50 ohms or smaller (so add some extra Rseries ~600 ohms at the input)
- for Rload between 50ohms-5kohms, the Rinput was between 0.83M-9.1Megaohms
- output impedance less than 0.5ohms (vs. ~15 ohms for a normal emitter-follower)
- gain voltage was 0.998 (vs. 0.984 for a normal emitter-follower)
- Rgenerator and Rload controls overshooting
At an AC simulation, the output impedance is less than 0.1\$\Omega\$ for a very broad frequency range. All these look almost too good to be true for one extra transistor, plus the circuit is not mentioned very often, so I wonder what's the catch.
Anybody tried using the White emitter-follower in practice, how well does it work?