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| Analysis of TC compensated Voltage Reference / Discrete Linear Regulator |
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| David Hess:
--- Quote from: AG7CK on June 03, 2018, 10:12:03 pm ---Thanks again. I think I understand a new point: The adjustment of / change in Ic is not because they "push" current (Vout - Vc)/Rc into the collector thereby increasing Ib. It is more like the "inverse" relation Ib=(1/Beta)xIc arises because adjusting Rc changes Vc which is the input signal to the diffamp. So a change in Rc marginally adjusts Vout and hence Vb and Ib? --- End quote --- No, the differential amplifier has the same voltage at both bases so the collector voltage of the error amplifier is fixed. Since the positive end of Rc is also fixed, Rc has a constant voltage across it and changing it only affects the collector current of the error amplifier. This has only an incidental effect on the output voltage and a significant effect on the temperature compensation of the zener diode. The reason these voltages are fixed is that the control loop is constantly adjusting them to match the voltage at the base of Q17 which itself is a fraction of the reference output voltage. The absolute value of this voltage is unimportant but it is important that it not change. The change in collector current alters the emitter resistance which alters both the transconductance of the error amplifier and its noise; neither is important. It also affects the base current which is also unimportant. The temperature coefficient on the other hand is important. |
| AG7CK:
--- Quote from: David Hess on June 03, 2018, 11:30:49 pm --- --- Quote from: AG7CK on June 03, 2018, 10:12:03 pm ---Thanks again. I think I understand a new point: The adjustment of / change in Ic is not because they "push" current (Vout - Vc)/Rc into the collector thereby increasing Ib. It is more like the "inverse" relation Ib=(1/Beta)xIc arises because adjusting Rc changes Vc which is the input signal to the diffamp. So a change in Rc marginally adjusts Vout and hence Vb and Ib? --- End quote --- No, the differential amplifier has the same voltage at both bases so the collector voltage of the error amplifier is fixed. Since the positive end of Rc is also fixed, Rc has a constant voltage across it and changing it only affects the collector current of the error amplifier. This has only an incidental effect on the output voltage and a significant effect on the temperature compensation of the zener diode. The reason these voltages are fixed is that the control loop is constantly adjusting them to match the voltage at the base of Q17 which itself is a fraction of the reference output voltage. The absolute value of this voltage is unimportant but it is important that it not change. The change in collector current alters the emitter resistance which alters both the transconductance of the error amplifier and its noise; neither is important. It also affects the base current which is also unimportant. The temperature coefficient on the other hand is important. --- End quote --- This solves long time on-and-off thinking about refamps for me. I will have to read more about transistor theory. To me the error amplifier / refamp seemed like a Common Emitter with a (almost) constant low impedance voltage source (the zener) in the emitter. And all I knew was Ib=Hfe*Ic. I have several of these refamp devices - including one that is running as reference in a Fluke 8505A 6.5-7.5 digit DMM. Even if I could just forget all about the refamps and use LT1021 (non-heated), LM399 and LTZ1000 (depending on need), it is a meaningful lost cause for me to build and (more importantly understand) a discrete reference and oven that matches first LM399 and then LTZ1000 in stability. All these 3 more modern references are TC compensated zeners (a zener in series with a PN junction). So a refamp circuit built from precision junk box parts in a stable external oven should be able to match them. Thanks a lot to both of you. |
| David Hess:
--- Quote from: AG7CK on June 04, 2018, 12:01:32 am ---This solves long time on-and-off thinking about refamps for me. I will have to read more about transistor theory. To me the error amplifier / refamp seemed like a Common Emitter with a (almost) constant low impedance voltage source (the zener) in the emitter. And all I knew was Ib=Hfe*Ic. --- End quote --- What really helps in these analysis is recognizing differential pairs used in closed loop feedback circuits like Q15 and Q17. When the feedback loop is closed, then the voltages between the bases are zero. This is the same rule where the inputs to an operational amplifier have zero volts across them and no current flows into or out of them. Of course these are all simplifications and there is a voltage and current does flow in or out of the inputs and in precision designs these need to be taken into account. --- Quote ---I have several of these refamp devices - including one that is running as reference in a Fluke 8505A 6.5-7.5 digit DMM. Even if I could just forget all about the refamps and use LT1021 (non-heated), LM399 and LTZ1000 (depending on need), it is a meaningful lost cause for me to build and (more importantly understand) a discrete reference and oven that matches first LM399 and then LTZ1000 in stability. All these 3 more modern references are TC compensated zeners (a zener in series with a PN junction). So a refamp circuit built from precision junk box parts in a stable external oven should be able to match them. --- End quote --- Those old references are surprisingly stable and high performance. Even now you can buy 5ppm/C temperature compensated zener diodes but their price and difficulty of use makes references like the LT1021, LM339, and LTZ1000 economical. |
| AG7CK:
Yes, it has occurred to me that the input of all op amps is a differential stage. Also, the next development in these reference boxes - the Fluke 731A - uses an identical circuit with an op amp instead of the "long tailed pair". I will be busy with life stuff a few days. When I come back, I will try to formulate a procedure for a build, selection of parts and adjustment / trim of one of these circuits. Thanks again. Very generous of you to spend all this time on the topic. |
| David Hess:
--- Quote from: AG7CK on June 04, 2018, 12:33:09 am ---Yes, it has occurred to me that the input of all op amps is a differential stage. Also, the next development in these reference boxes - the Fluke 731A - uses an identical circuit with an op amp instead of the "long tailed pair". --- End quote --- Differential pairs are the king of precision and integrated differential pairs are both the best and most economical so it is not surprising that Fluke replaced that discrete differential pair with an operational amplifier even though there was nothing special about the 2N3391. With operational amplifiers being as good as they are, other circuit considerations and especially the reference itself are the primary contributors to errors. |
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