why diodes are connected in serial?

In analog schematic, I often find few diodes are connected in serial.
Is this methods for voltage drop or for sth else?

Yes, sometimes it is for voltage drop. Other times it is to increase the reverse breakdown voltage.

yes right it's to manipulate the voltage without influence the current. coz the diode can block the voltage by 0,2-0,7 (if i'm right)

EDA_hg81,
Diodes are often connected in series in biasing circuits so that their combined voltage drops will track the base-emitter voltage over temperature.
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It is not a good idea to connect diodes in series to provide an increased reverse voltage rating. The reason is that if a voltage with a small rise time is applied, the voltages across the diodes will distribute themselves in inverse proportion to their capacitances. As a result, the diode with the smallest capacitance will have the largest voltage applied. Let C1 be the capacitance of CR1; C2 be the capacitance of C2. Then the voltage across CR2 will be V[C1/(C1 + C2)]. As an extreme case, if C2 = 0, then the voltage across CR1 would be V.
Regards,
Kral

Added after 1 minutes:

I meant to say "C2 be the capacitance of CR2"

Thank you all.

hey if it's a class B amplifier or something similar you're looking at, it's in there for crossover distortion prevention, in which case the 2 diode drops are suppose to match the 2 output transistors base-emitter junction.

To connect diode in series to provide voltage drop to bias for BE of transistor, but why don't use resistor? because diode depend on temperature, when temperature is higher, voltage drop is smaller which cause current of transistor is decrease.

In standard CMOS process, it is not likely to have series diodes,is that right?
for example, you made the Pdiff-Nwell diodes, and wants to put two of them series. but the parasitic PNP trs already shorts the two series diodes as one.

In HVCMOS process, I find someone use deep nwell to do the series diodes.
but the reverse voltage of such diodes is small.

Does someone hnow in what kind of process , the series diodes can be made safely? That is , if I want to get series diodes in a process, what characteristics the process need to have?

maybe used as a clamp as in HV CMOS to protect gates.

This is when you need about n*0.7V of voltage drop, where n is the number of diodes.