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There is an example in the book 'The art of electronics - 3rd edition' that I can't understand. The issue is: How the voltage 0.6V goes to the diode D2 without a drop across R1.

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This question is addressed here: Compensating the forward voltage drop of a diode signal rectifier. But my main doubt (how 0.6V goes to the anode of D2) isn't dealt with.

In my opinion there must be some voltage across R1 and because of that the voltage 0.6V don't "flow" to the anode of D2.

Am I missing something here? The sinewave signal injected in 'in' doesn't generate a drop across R1? How the voltage that comes from 'in' can be added with the voltage on top of R1? If 'in' is high, e.g. 10V, what is going to happen?

In mind mind, I cannot figure how a voltage on top of R1 can be added to the voltage coming from 'in'. If 'in' is 10V the anode of D2 is going to be 10V and a voltage drop will appear across R1 (10V - 0.6V).

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  • \$\begingroup\$ You say that voltages flow, that's a big no no. Currents flow through components, voltages 'sit' on nodes. To understand your circuit, try to look at R3 D1 as a 0.6V voltage generator. \$\endgroup\$ – Vladimir Cravero Oct 27 '16 at 12:22
  • \$\begingroup\$ I used 'flow' because of how this issue is addressed in a comment in another question that explains this circuit. electronics.stackexchange.com/questions/68019/… \$\endgroup\$ – Rodolfo Oct 27 '16 at 12:25
  • \$\begingroup\$ I saw the linked question. It seems to me the analisys is pretty good there. Have you tried to solve the DC circuit? Remove the capacitor, assume D1 is forward biased, compute current through R3. Now assume a drop of 0.6V across D2 and compute the current through R2, it is a simple eqn system in Ir2 and Vout. What can you say now? How doeas the small signal circuit look? \$\endgroup\$ – Vladimir Cravero Oct 27 '16 at 12:35
  • \$\begingroup\$ Did you not read my explanation (and the comments below) from the question you linked? \$\endgroup\$ – Andy aka Oct 27 '16 at 13:15
  • \$\begingroup\$ What will happen at the instant when in is at 10V? \$\endgroup\$ – Rodolfo Oct 27 '16 at 13:49
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re: "In my opinion there must be some voltage through R1 and because of that the voltage 0.6V don't "flow" to the anode of D2.

Am I missing something here? The sinewave signal injected in 'in' doesn't generate a drop through R1?"

The valid current paths are:

A. R3-->D1-->gnd
B. R3-->R1-->D2-->R2-->Gnd
Note: D2 must be forward biased (i.e. conducting) for current to flow through R2

The voltage drop across R1 will depend on the input signal voltage. When the input signal V is too low, D2 will not become forward biased.

There is .6V at the bottom of R1. If the +in V is high enough, it will add to the voltage that has passed through R1 and the combined V will be sufficient to cause D2 to become forward biased. Once D2 is fb, current will flow through R2 to gnd.

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Look at that arm of the circuit : and assume you have 0.6V across R1, D2 and R2, all in series to ground.

Now, what voltage is dropped across R1, what is dropped across D2, and what is dropped across R2?

You know that the voltage across R2 is 10x the voltage across R1, and that diode D2 won't start conducting below 0.6V across it. Therefore virtually the whole voltage will be dropped across D2, leaving very little across R2, and 1/10 of that across R1.

So, to a fairly close approximation, the anode of D2 must be at 0.6V.

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  • \$\begingroup\$ "the whole voltage will be dropped across D2". Isn't the maximum voltage across the diode 0.6V? \$\endgroup\$ – Rodolfo Oct 27 '16 at 13:05
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Ther is a bunch of current (about 4.3mA) flowing through R3, most of which flows through D1. For DC, D2 is effectively in parallel with D1 with 1K + 10K = 11K in series, so only a bit of current flows through D2, however in reality it won't be zero.

You can solve the Shockley diode equation or do a simulation to find the DC voltage at the output, it will probably be a couple hundred mV, and the voltage drop across R1 will be 1/11 of that or a couple tens of mV.

Let's do a simulation:

schematic

simulate this circuit – Schematic created using CircuitLab

According to this, output voltage is 218mV and thus the voltage across R2 is about 20mV.

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