Diodes at the base of PNP transistor - Page 1
Forum section: electronics / beginners

author: JeanF
author_rank: Regular Contributor
timestamp: April 29, 2020, 11:31:02 am
content: Hi,I took apart an old OCXO to see what's inside, and train myself to "read" what circuit blocks do. I'm still studying the oscillator parts and I will probably ask questions later, but for now here is the thermostat circuit :Could anyone give me a tip about what D2 and D3 do ? I think they clamp the base voltage to about ~10.6 V, which would also set Ve to 11.3 V, so that would set the emitter current.- Is this correct ?- Why use diodes instead of just using another value for R8 and no diodes, that would give the same operating point when the output of the Opamp is low ?Speaking of playing with component values, I doodled this part of the circuit in LTSpice, which simulates without any problem, of course. The 0.2 V source represents the output of the opamp when low.The model is not 100% accurate, because I've used generic diodes instead of 1N4154s and because treating the output of an Opamp as a perfect voltage source is probably not a good thing, but that's not my point. What if I wanted to solve this (approximately, at least) by hand, would it be possible? What assumptions can I safely make, and which should I make first? For example if I say that one diode drop is 0.7 V and that Ve - Vb is 0.7 V as well, I can calculate all the remaining unknown voltages and currents, but then the emitter and base currents, for example, are 60 to 70% off compared to LTSpice results. Of course I have much more confidence in simulator results than in mine! But for the sake of learning, would there be a better manual way?Thank you very much,JF

author: Benta
author_rank: Super Contributor
timestamp: April 29, 2020, 11:55:01 am
content: They're for limiting the current through Q6. As you say, one diode equals VBE and the second makes sure that the voltage across R10 and R11 can't go higher than 0.7 V.

author: JeanF
author_rank: Regular Contributor
timestamp: April 29, 2020, 01:36:58 pm
content: Thank you.Is it a belt and braces kind of thing, or is this necessary ?If we remove the diodes and change R8 to 820 or 910 ohms, that seems to limit the emitter current to about the same value (around 325 mA). What could go wrong in that case ? The opamp is a TAA762 by the way.The current could increase if the value of R6 drops with age maybe, but is this a common thing ?

author: Benta
author_rank: Super Contributor
timestamp: April 29, 2020, 06:36:01 pm
content: No, the diodes cannot be removed.As a quick estimate, the current through Q6 could increase to ~1.8 A, if the output voltage of the op amp is close to zero.Your suggestion of changing the resistor values would make this worse.When the circuit is operating in stable conditions, they may not be necessary, but in transient conditions (power up, power down, playing with VR1) they do have a function.This seems to be the heater circuit for the oscillator, with Q6 being the heater. Right?

author: JeanF
author_rank: Regular Contributor
timestamp: April 29, 2020, 09:39:07 pm
content: Quote from: Benta on April 29, 2020, 06:36:01 pmQuoteThank you again !much appreciated. That does indeed make sense.Quoteyes, right ! I bought that thing long ago on ebay for cheap (and for no good reason), I think it's been removed from a W&G SPM-19.Quotesorry to be such a noob again, but how ? Could you please give more detail?I don't get anywhere close to 1.8 A with pen and paper. Assuming the gain of Q6 is 100, applying the node voltage method at the base of Q6 and solving for Vb, I get Vb = around 10.5 V, which gives me a collector current of about 370 mA. And reducing R8 while keeping R6 the same as before makes Vb go up and Ic go down... LTSpice gives a bit less (more like 350 mA) because it doesn't keep Vbe at 0.7 V, it lets it go to about ~0.9 V. Did I goof up the numbers?Numbers aside, I'd still be very interested to know how you do your quick estimate. That's one of the things I find difficult while learning alone, I don't have anyone to show me tricks... Thankfully forums are greatI thought that with a bipolar opamp the output couldn't go below about 0.2 V, is that true ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 29, 2020, 11:03:02 pm
content: Wider operating voltage range. As is, maximum power is still proportional to supply, because maximum current is limited approximately constant but power is the product of voltage and current. If a resistor divider were used, power would go as the square of voltage.Tim

author: Benta
author_rank: Super Contributor
timestamp: April 29, 2020, 11:24:04 pm
content: The estimation for the current through Q6 is fairly easy, assuming the diodes are removed. In an extreme situation, where the op amp output is ~0 V, the base voltage at Q6 is 8 V. Meaning that base voltage is -4 V as seen from Q6. The voltage across R10/11 is then ~3.3 V, resulting in an emitter current of over 1.5 A.I estimated 1.8 A formerly, but that was just a very quick calculation.WITH the diodes, maximum current through Q6 is around 325 mA.Concerning my estimates, a lot is experience.But I have some rules:NEVER assume anything about hFE. Always use best/worst case (10 or 100000). Do not use hFEfor circuit analysis/synthesis, but use ideal transistors. You can always use the real parameters later.ALWAYS do a reality check (this applies to everything in life). Is this voltage realistic? Is this current realistic? If you expect uA, why is it suddenly mA?Do your analysis on paper. So many students rely on LTSpice or equivalent. Excellent tools, but the wrong starting point.

author: Benta
author_rank: Super Contributor
timestamp: April 29, 2020, 11:28:02 pm
content: Quote from: T3sl4co1l on April 29, 2020, 11:03:02 pmAre you certain you're in the right thread?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 29, 2020, 11:40:11 pm
content: Are you..? It's a heater, so power and range are important... Voltage range probably not important, but that's a motivation behind that type of circuit.Tim

author: JeanF
author_rank: Regular Contributor
timestamp: April 30, 2020, 10:28:11 am
content: Thank you both