#1279 Three Transistor Voltage Regulator

Comments · 201

• W2

  @w2aew · 3 years ago

  My analysis process was a bit different.  By observation, there is 13.1V across the 13.7k resistor, so you can calculate the current in the two resistors (ignoring the base current).  Then, calculate the IR drop across the 4.32k, and add 0.6 to that.

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• AD

  @aduedc · 3 years ago (edited)

  Here are a few points:<br><br>1- The diode is there to make sure if there is dip in the output voltage -- as a result of transient --, it won&apos;t go below - 0.7V , that will save your C789 (39uF Cap), without that your Cap will get reversed biased.<br><br>2- &nbsp;you can calculate the current through R783 is equal to &nbsp;( 0.6 - (-12.5))/13.7K <br><br>3. 0.6V at the base of Q780 is our kinda reference in this feedback loop (Variation in hfe of transistors or load current does not change this base voltage, only temperature will)<br><br>4. &nbsp;If you open the feedback loop and do loop analyzes, you will realize R788 (which you taken off is there to stabilize the feedback loop)<br><br>5. Your open loop gain of the feedback loop is gm(of Q780) * R780 * (R783/(R783+R782))<br><br>6. &nbsp;Your dominate pole (1st pole) in this feedback loop is1/{2*PI* [(R788+ 1/gm(of Q787))*C789]}<br><br>7. &nbsp;gm pf any BJT transistor is Ic/VT, VT is about 0.026 V &amp; Ic is the collector current<br><br>8. &nbsp;Your first zero &nbsp;of feedback loop is 1/(2*PI*Resr*C789); &nbsp;Resr is the esr resistance of C789<br><br>9. &nbsp;Your parasitic pole of the loop is 1/(2*PI*R780* C_parasitic of the Base Q787) diffusion cap of Q787 C~Cb=tf*gm &amp; tf =W^2/(2*Dn) &nbsp;W is effective base &amp; thickness Dn &nbsp;is Diffusivity<br><br>10. Q784 &amp; R788 (which you eliminated) act as short circuit protection.<br><br>11. &nbsp;The advantage of this Voltage Source over regular LDO:<br> &nbsp; &nbsp; &nbsp; &nbsp;A. &nbsp;It has its own voltage reference. <br> &nbsp; &nbsp; &nbsp; &nbsp;B. &nbsp;Does not require opamp<br> &nbsp; &nbsp; &nbsp; &nbsp;C. &nbsp;Maybe can be designed to be faster ( higher ZGF and still stable)<br> &nbsp; &nbsp; &nbsp; &nbsp;D. &nbsp;Lower over all transistor count.<br> &nbsp; &nbsp; &nbsp; &nbsp;F. &nbsp;Maybe can be can be designed to have lower noise (Bandgap reference noise and opamp noise is eliminated, and we can optimize this type of voltage source for low noise)<br><br>Anyways, interesting circuit thanks for pointing it out.

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• VI

  @vincei4252 · 3 years ago

  but I guess the circuit depends on -12.5V rail be regulated with respect to ground all other things being equal.

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• XD

  @xDR1TeK · 3 years ago

  There is beauty in little things after all.

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• AK

  @akekarlsson2612 · 3 years ago

  You makes it so easy to understand electronics. You guide in a very methodically and calm way.<br>Thank You!

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• BR

  @bradhernlem1548 · 3 years ago

  Tektronix had the best electronic drafting style!

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• ST

  @stevehanlon7627 · 3 years ago

  fun little circuit analysis, thanks for taking us along.

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• BI

  @bigjd2k · 3 years ago

  Simple and accurate enough! And repairable.

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• PL

  @ploegmma · 3 years ago

  Very interesting. With your analysis and explanation it all sounds obvious. But it would have taken me a long time to fully figure it out. Great content!

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• EM

  @emptech · 3 years ago

  Back in the days, in the 60&apos;s when I was a student, we would check out the service manuals from HP and Tek. &nbsp;There would be a very good explanation as to how the circuits worked. &nbsp;You don&apos;t get that anymore. &nbsp;HP even made it more and more difficult to get diagrams. &nbsp;I always preferred Tek over HP. &nbsp;I think the Tek stuff had better human engineering too.<br><br>Gee, why didn&apos;t they just use a 7805, they didn&apos;t exist them. &nbsp;They did have op amps though. &nbsp;Don&apos;t think there were 723&apos;s either.<br><br>Thanks, very good. &nbsp;Jim

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• CA

  @caffesenza5836 · 3 years ago

  Thanks for presenting this piece of old school electronics, very interesting. <br>I think of it this way: it is an opamp in inverting configuration whose gain is -R782/R783 and whose input referred offset is the Vbe of Q780. They use their negative rail as a input to this inverting amp.

• JO

  @joelstyer5792 · 3 years ago

  Old Tek manuals are one of the best places to learn good electronic design. Sometimes they over complicated things a lot but usually for good reason. The best one I ever read was the Tek 650 video monitor schematic. The circuit design is excellent, but they also put humorous little drawings in for different circuits, i.e. near the back porch generator circuit was a drawing of a generator sitting on a back porch. I seem to recall there were about a half dozen of these which always cracked me up.<br><br>The reason the voltage calculated low was that the base of Q780 will bleed a tiny bit of current through R782. Knowing Tek, I bet this was all highly calculated for all temperature and current ranges. The stuff they built from this era generally had very robust designs.<br><br>For those wondering how it regulates so well, I was going to go through it but there is a great explanation in two comments from Rex Schneider near the bottom of the thread. It is that -12/5V supply that is critical to the proper operation.<br><br>Great video, I grew up with this stuff, so it is nice to see someone explaining how these circuits work. Still very valuable knowledge to have these days.

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