What is the puporse of this Diode in parallel with a resitor in a base of Transistor?

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Thevenin

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Hello All,

Please, what is the purpose of D14 in this circuit?



I can't realize it's function.

Thank you.
 

Normally such diode resistor network is used to have fast turn-on of the transistor (as current goes through the diode), and slower turn-off as base charge is removed via the resistor.

In your situation it looks strange. The emitter is at 0V. when the IC outputs 3.3V (just an example), the base current is determined by almost the short circuit current of the CPU output. Can it be an open collector/drain output with internal pull-up resistor? If so, then it makes sense.
 


It's from a ZXSpectrum 2

ftp://ftp.worldofspectrum.org/pub/sinclair/technical-docs/ZXSpectrumIssue2-Schematics.gif

I don't know if is it a open collector output.

I can't understand this part of the circuit.

Looks like a cut off of 5V Power if there is a 0 in CPU output, maybe?
 

Looks to me like it's the clock output from the ULA to the Z80. It needs fast clock edges so a speed-up diode wouldn't be a bad idea but unless the ULA has something limiting it's output current it looks as though it would be overloaded. These were very much cut price machines so if the schematic is correct, maybe they do it just to cut costs even though it's considered bad practice.

Brian.
 

I still remember the diode is used for the stability of Icbo
 

Icbo is the transistor leakage current in it's unbiased state so I can't see that would have any effect. The problem is the output of the ULA would see the diode and B-E of the transistor as two Si junctions to ground and would try to sink excesive current into them. Either the ULA has an internal current limited output or more likely, the schamatic is wrong and the diode should face the other way. If the diode is reversed, it would have the desired effect of speeding up the clock edges. Z80 devices need fast clock edges to work properly. The Z80 data sheet shows a similar transistor assisted clock drive.

Brian.
 

the r26 can protect the CPU pin from surge voltage,and the diode D16 provide the path to discharge to the GND.
 

??? R26 is in series with the Z80 NMI signal - what has that got to do with surge protection?
??? D16 is the negative 5V regulator to the DRAMs

What has this got to do with the original question?

Brian.
 

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