# Diodes with Transistors

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#### PrescottDan

##### Banned What does these diodes do with the transistor?  #### Audioguru In the top circuit with the 3 diodes to ground:
1) Are the diodes forward or reverse biased?
2) Do the diodes conduct current?
3) Roughly how much voltage will be across the 3 diodes?
4) Does the voltage across the diodes increase or decrease when something heats them?
5) If the base voltage of the transistor decreases a little does the transistor conduct more or conduct less?
6) Does the transistor conduct more current or conduct less current when something increases its temperature?

In the second circuit;
1) If the input is 0V then does the transistor conduct or not?
2) if the input is positive a few volts then does the transistor conduct or not?

#### PrescottDan

##### Banned In the top circuit with the 3 diodes to ground:
1) Are the diodes forward or reverse biased? Depends on the Input polarity?
2) Do the diodes conduct current? Yes and conduct voltage
3) Roughly how much voltage will be across the 3 diodes? depends on the input voltage
4) Does the voltage across the diodes increase or decrease when something heats them? Increases , current increases
5) If the base voltage of the transistor decreases a little does the transistor conduct more or conduct less? conducts less
6) Does the transistor conduct more current or conduct less current when something increases its temperature? conduct more current

In the second circuit;
1) If the input is 0V then does the transistor conduct or not? NO, it's off
2) if the input is positive a few volts then does the transistor conduct or not? Conducts

#### Audioguru In the top circuit with the 3 diodes to ground:
1) Are the diodes forward or reverse biased? Depends on the Input polarity?
Since the NPN transistor has a positive supply and ground then why would the input ever be negative?

2) Do the diodes conduct current? Yes and conduct voltage
Yes a forward-biased diode conducts current but no it does not conduct voltage. It has a forward voltage drop.

3) Roughly how much voltage will be across the 3 diodes? depends on the input voltage
Not much. The typical forward voltage of a conducting 1N4148 silicon diode is about 0.66V at 2mA and is 0.70V at 5mA and is 0.73V at 10mA.

4) Does the voltage across the diodes increase or decrease when something heats them? Increases , current increases
No, when a diode temperature increases then the forward voltage decreases a little exactly the same as the base-emitter diode in a transistor.

5) If the base voltage of the transistor decreases a little does the transistor conduct more or conduct less? conducts less
Yes.

6) Does the transistor conduct more current or conduct less current when something increases its temperature? conduct more current
Yes.

In the second circuit;
1) If the input is 0V then does the transistor conduct or not? NO, it's off
Yes.

2) if the input is positive a few volts then does the transistor conduct or not? Conducts
No. The input diode will never conduct because it is backwards.

It appears that you know some things about diodes and transistors but do not know many other basics about them. You should go back to school.

• KhaledOsmani

### KhaledOsmani

Points: 2

#### PrescottDan

##### Banned Since the NPN transistor has a positive supply and ground then why would the input ever be negative?

Because the Input signal can be a AC sinewaveform, triangle, or squarewaveform

They use DC offset to raise the DC above the ground so the AC sinewaveform can swing positive and negative without getting clipped off

you don't need a negative power supply, just use DC offset

Yes a forward-biased diode conducts current but no it does not conduct voltage. It has a forward voltage drop.

So a diode "blocks" voltage and only conducts current? the diode converts a voltage to current?

Rectified diodes convert an AC voltage into a DC voltage, what you mean a diode doesn't conduct voltage only current?

No, when a diode temperature increases then the forward voltage decreases a little exactly the same as the base-emitter diode in a transistor.

Why does a diode decrease in voltage when the diode or transistor gets hot or increases in temperature?

So when a diode or transistor get hot, the current and voltage is lower that is passing through the diode or transistor and on the transistors output the current is lower?

#### Audioguru Because the Input signal can be a AC sinewaveform, triangle, or squarewaveform
They use DC offset to raise the DC above the ground so the AC sinewaveform can swing positive and negative without getting clipped off
you don't need a negative power supply, just use DC offset
With a positive DC offset then the base of the transistor never goes negative. Then the input is positive or 0V.

So a diode "blocks" voltage and only conducts current? the diode converts a voltage to current?
Of course not. A reverse biased diode conducts no current. A forward biased silicon diode conducts current and has a voltage drop of about 0.7V.

Rectified diodes convert an AC voltage into a DC voltage, what you mean a diode doesn't conduct voltage only current?
A rectifier or a rectifier bridge converts AC current into DC current. The rectifier or rectifier bridge passes most of the forward voltage minus the diode voltage drop.

Why does a diode decrease in voltage when the diode or transistor gets hot or increases in temperature?
It does not matter why. Every datasheet shows it.

So when a diode or transistor get hot, the current and voltage is lower that is passing through the diode or transistor and on the transistors output the current is lower?
Absolutely NOT!
When a diode or transistor is heated then its forward voltage drop decreases a little (then the output voltage of a diode increases a little) and the transistor current gain increases which allows it to conduct more current. It is shown in every datasheet.

#### PrescottDan

##### Banned With a positive DC offset then the base of the transistor never goes negative. Then the input is positive or 0V.

So how do you get a waveform to swing positive and negative on the input to a transistor and output of a transistor when there is only a positive power supply to ground, not negative power supply?

It's Positive DC offset

Of course not. A reverse biased diode conducts no current. A forward biased silicon diode conducts current and has a voltage drop of about 0.7V.

Rectified diodes convert an AC voltage into a DC voltage, what you mean a diode doesn't conduct voltage only current?
A rectifier or a rectifier bridge converts AC current into DC current. The rectifier or rectifier bridge passes most of the forward voltage minus the diode voltage drop.

Then my Oscope would need a current probe to view the waveform

I use a normal Oscope probe and i can view voltage waveforms before and after a diode

A diode "passes" voltage and it pass a waveform that is a voltage, not a current waveform , i would need a current probe

Conducting means it "passes" current and voltage

When a Transistor is turned OFF or no output on the collector, the collector is High Impedance?
When a transistor is Turned On or has an output on the collector, the collector is Low Impedance?

When a transistor has no input on the base, the base is high impedance?
when a transistor has an input current on the base, the base is low impedance? So how do you get a waveform to swing positive and negative on the input to a transistor and output of a transistor when there is only a positive power supply to ground, not negative power supply?
Using a coupling capacitor in input, which will feed the AC into offset present in the base of the transistor.

Then my Oscope would need a current probe to view the waveform
No. Start with a single circuit to get it better. There is a lot to learn with a transistor in a circuit.

When a Transistor is turned OFF or no output on the collector, the collector is High Impedance?
When a transistor is Turned On or has an output on the collector, the collector is Low Impedance?

When a transistor has no input on the base, the base is high impedance?
when a transistor has an input current on the base, the base is low impedance?
You cant ask and say in that way, Input and output impedance of amplifiers not changing with amplitude of given input signal.

I suggest you to start with capacitor diode circuits to understand electronics better, like clipper, clamper, Voltage doubler. A circuit with transistor is just full of calculations, nothing more to understand.

#### SunnySkyguy 