Basic transistor question

[leto]

Hello,

I´d appreciate your help with the following schematic. I don't understand why the voltage drops 1,87v between the base and GND?. I thought it should be 3,16v - 0,7v (diode drop) = 2,46v. And why changing R3 effects such value. For example, setting R3=1K, the voltage goes to 2,92v.

thanks in advance

 

[Relayer]

The 10k (R3) resistor has it's own voltage drop across it: 3.16 - 1.87 = 1.29 volts.
When you're measuring from Base to Ground, it will be 1.87 volts: 1.87 - 1.28 = 0.59 which is the Diode drop.
Remember R2 will have it's own voltage drop as well.
Regards,
Relayer

 

[KlausST]

Hi

In your case:
3.16V is on the left side of R3.
As soon as there is current through R3 there will be a voltage drop across R3.
Now the base current is not zero, thus R3 current is not zero.
Usually the BJT "amplifies" the base current ... in your case this is impossible, because your voltmeter in series with the collector prevents current flow.
(This is not how a voltmeter should be connected. This is not like a bjt should be connected)
Since there is no collector current all emitter current needs to be supplied through the base.

To make it work like usual:
* connect the collector to 9V battery, or
* replace the collector voltmeter with an Amperemeter.
Then there will be collector current. Now the bjt is able to "amplify". The emitter current will increase, but the base current will decrease.

I expect just 2...3uA of base current.thus the voltage drop across R3 will be reduced and the voltage base_to_GND will be almost 3.16V.

Mind: a bjt can not "generate" current. When we talk about "it amplifies the base current" then it means it takes most of the current from collector.
(Only if possible ;-) )
**********

Generally:
[corrrected after post#4: The greyd out statement is not clear, thus please read post#4 and post#5]
When one talks about "base voltage" then usually V_BE = Voltage between Base and Emitter is meant.
This also is the value given in every bjt datasheet.


Unlike other components (like ICs) bjts have no dedicated GND pin.
When a part has a GND pin then the datasheet usually says something like this: "All voltages are referenced to GND, unless mentioned otherwise.)

Klaus

 

[LvW]

When one talks about "base voltage" then usually V_BE = Voltage between Base and Emitter is meant.
This also is the value given in every bjt datasheet.

Just a short comment:
When I mention the term "base voltage" I mean the voltage at the base node (against ground, of course). This voltage is not identical to V_BE if there is an emitter resistor (which is the case in nearly all amplifier applications, in particular for emitter followers)).

 

[KlausST]

Oh, a misunderstanding.

I should have been more clear.
LvW, you are right. The same is with me. When I talk about base voltage, then I mean base-to-GND voltage, too.

What I meant was: when if "hobbyists" sloppily talk about base voltage (often seen here in this forum) then they often mean V_BE.

The correct terminology is:
base voltage = base to GND voltage
V_BE = base to emitter voltage (as given in the datasheet)

Thanks for pointing that out.
(to avoid confusion I´d correct my statement above)

Klaus