Wide band discrete current source/sink; long tailed pair, cascode performance? - Page 1
Forum section: electronics / projects

author: evb149
author_rank: Regular Contributor
timestamp: January 09, 2022, 11:48:32 pm
content: Hi -- I'm wondering what kind of wide band results people have gotten from prototypingbipolar or FET based :* long tailed pair differential amplifiers and also (independent circuits but also of interest as driving the long tailed pair),* current sources / sinks,* cascode circuits (relevance to above e.g. Wilson cascoded current mirror performance vs. others)...for fairly wide band use (e.g. DC-Ghz sort of response being interesting)The circuitry is simple enough to prototype / test but I thought I'd get some anecdotes about who may have built / tested suchwith various sorts of transistors and how the test results came out, whether there were significant instability issues being discrete, etc.

author: mawyatt
author_rank: Super Contributor
timestamp: January 10, 2022, 01:04:04 am
content: Here's one we used long ago with a complementary bipolar process (9.6GHz NPN & 4.8GHz PNP). Simple to build and test with a couple 2N3904s and 6s.https://www.edn.com/peaking-current-source-has-high-rejection/Best,

author: jwet
author_rank: Frequent Contributor
timestamp: January 10, 2022, 03:04:01 am
content: A couple of small inputs- rich topic and please post your results, this low level design stuff is what I enjoy most.A technique that IC designers used back in the old days (70's) was to breadboard circuits at scaled down frequencies- usually ten to one or more. I know National did this. A lot of those crazy rats nets breadboards were scaled this way.A little hacker company called Evil Mad Scientist Labs makes discrete kit versions of classic linear IC parts like the uA741 Op-Amp. Might be a good place to start.I've done some pretty high frequency stuff on solderless breadboards- over 150 MHz. You have to keep in your mind that there are 3 pF caps between everything and their reactance at 100M is 500 ohms. One circuit I remember was a frequency converter circuit that used an NE=602 and a 16 Mhz crystal to bring the NOAA weather band (162.xx) into the middle of the 2 meter (146.xx) ham band. I was kind of amazed myself.In Jim Williams analog design books- The first The "Art and Science of", there is a contribution by James Bryant about Analog Breadboarding- he gives a bunch of practical guidelines and then does mostly Manhattan type construction on copper clad ground plane and some modular Manhattan stuff for using IC's. Good advice.Both of the Williams' book are a great read if you're thinking about these kinds of things. Jim himself is joined by about a dozen contributors in both books- real luminaires- Paul Brokaw, Phil Perkins, Doug Grant, Richard Burwen, Derek Bowers, Bob Pease, Barry Gilbert, etc.

author: moffy
author_rank: Super Contributor
timestamp: January 10, 2022, 05:45:24 am
content: The discrete Wilson mirror doesn't work too well at very high frequencies, as it is fairly high impedance around the bases of the mirror transistors, which makes it a high accuracy mirror but lowers the frequency response. They have trouble getting current out of the mirror base fast enough.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 10, 2022, 02:56:58 pm
content: Ouput impedance plummets at higher frequencies becasue of shunt impedances in the current pass element.Eg a 10GHz transistor will have say 0.5pF of Cob. Or 300 Ohms at 1GHz.

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2022, 06:36:03 pm
content: Simple current sinks are useful up into the GHz range where impedances are low anyway. They can be depletion mode FET current sinks or the collector of a bipolar transistor. I am not sure I have even seen a cascode added except at lower frequencies where output capacitance does not dominate the output impedance.

author: TimFox
author_rank: Super Contributor
timestamp: January 10, 2022, 08:26:18 pm
content: As discussed above, at high frequencies the high shunt impedance of a constant-current circuit is limited by its capacitance.At a different frequency range, three years ago I investigated using a simple constant-current two-terminal circuit as a plate load in a grounded-cathode circuit, or in a cathode load on a cathode follower circuit, using 7586 Nuvistor triodes operating at about 75 V and 8 mA. The circuit was based on the IXYX IXCP10M45S constant-current device, which is a glorified depletion-mode high-voltage MOSFET in a TO-220 package.I measured the DC characteristics (as a function of the control resistor in the source terminal), the AC noise in the audio range, and the dynamic shunt impedance. The DC operating condition for impedance and noise measurements was 7.4 mA and 66 V.To make a two-terminal component without additional fixture capacitance, I kept the central lead (drain or positive node) horizontal, while bending up the other two. A short insulated stand-off was screwed into the TO-220's tab and connected to the gate (through a 1 k resistor) and to the source (through Rset = 458 ohms), with an extension wire to form the negative node.1. Apparent DC resistance. This was affected by the slow warm-up of the device (without heatsink). Over the range from 40 to 85 V, waiting for warm-up at each voltage, the slope corresponded to 281 k. The highest power there was 730 mW.2. Dynamic (AC) impedance. With 66 V and 7.4 mA DC bias, capacitively coupling an AC generator and differential preamplifier gave an apparent impedance of 3.75 megohms in parallel with 18.2 pF, for a corner frequency of 2.3 kHz3. Broadband noise: with the generator disconnected, and defining the bandwidth by the filters in the PAR 117/114 preamplifier, gave 3.2 nA rms between 10 Hz and 300 Hz, and 4.1 nA rms between 300 Hz and 30 kHz. The broadband noise voltage was higher than that of a resistor, so I used the FET circuit in the plate of the second triode and the cathode of the third triode, but not at the first triode plate (after doing a detailed analysis of PSRR). The high impedance of the FET circuit helped the linearity of the second triode (with more plate swing than the first triode) and PS hum rejection of both the second and third triodes, but I had to use extra bypass capacitance feeding the plate resistor of the first triode to reduce PS-induced hum.

author: moffy
author_rank: Super Contributor
timestamp: January 10, 2022, 10:28:53 pm
content: Quote from: Terry Bites on January 10, 2022, 02:56:58 pmColour me impressed! I ran a simple LTSpice sim on a Wilson current mirror with 2N3904 BJTs, a 1k input impedance and 50 ohm output with suitable bias, and it was pretty flat up to 50MHz.