Can anybody explain about how bjt works??

[Herschel Peeler]

What about a current mirror. Doesn't this use a voltage to program a current?
Thanks
Adam

An excellent example. A current in is mirrored to a current out. They call it a current mirror not a voltage mirror. But yes, the voltage across one side does cause an equal voltage on the other side which does, in deed, cause a specific current out the collector. The voltage is incidental. The cause at work is the current.
Knowing the voltage you can not calculate the resulting current, can you?
Is there a graph on a data sheet that shows what base voltage results in some collector current?
Knowing base voltage what can you calculate? hOE?

 

[Audioguru]

An emitter resistor adds negative feedback, reducing the voltage gain and reducing non-linearity (reducing distortion).

 

[(*steve*)]

At which point you're discussing how particular circuits using transistors work, the same applies to other devices such as MOSFETs, jfets, and even vacuum tubes.

 

[Ratch]

Hmmm. Does that work for the base resistor also. The emitter resistor configuration isn't used very often. I just don't see that having a very profound effect. I'll have to play with the idea some.

The larger the emitter resistance is, the more linear will be the Ic curve. Sedra and Smith recommends 10 mV max for the signal voltage.





Ratch

 

[Herschel Peeler]

What about a current mirror. Doesn't this use a voltage to program a current?
Thanks
Adam

Good example. CURRENT mirror. Not voltage mirror. Yes the voltage dictates the collector current but the voltage is incidental. Can we calculate the collector current knowing the base voltage? The voltage is the result of current through the base, not the cause. When we pull current through a resistor a voltage develops across it. Why do you look at silicon any different? he voltage does not cause the current flow. Or does it?

Hmmm.

Okay, yes it is the voltage. We can have multiple secondary transistors. Each will, in theory, have the same collector current. The primary current does not change. Good point!
Where does this show up on a data sheet?

 

[davenn]

When we pull current through a resistor a voltage develops across it. Why do you look at silicon any different? he voltage does not cause the current flow. Or does it?

The current flows through a resistor BECAUSE of the potential difference across it .... no potential difference, no current

 

[Herschel Peeler]

The current flows through a resistor BECAUSE of the potential difference across it .... no potential difference, no current

Oh no! You are starting to make sense.

Knowing -E-B voltage we cannot predict collector current. Collector current depends on load resistance. What can we predict knowing base voltage?



"The gm of bipolar small-signal transistors varies widely, being proportional to the collector current. It has a typical range of 1 to 400 millisiemens. The input voltage change is applied between the base/emitter and the output is the change in collector current flowing between the collector/emitter with a constant collector/emitter voltage.



The transconductance for the bipolar transistor can be expressed as

g m = I C V T {\displaystyle g_{m}={\frac {I_{C}}{V_{T}}}} g_{m}={\frac {I_{C}}{V_{T}}}

where IC = DC collector current at the Q-point, and VT = thermal voltage, typically about 26 mV at room temperature. For a typical current of 10 mA, gm ≈ 385 mS.



The output (collector) conductance is determined by the Early voltage and is proportional to the collector current. For most transistors in linear operation it is well below 100 µS."



Do we find this on a transistor data sheet under Admittance?

Can we calculate this somehow from base voltage?

 

[Arouse1973]

Good example. CURRENT mirror. Not voltage mirror. Yes the voltage dictates the collector current but the voltage is incidental. Can we calculate the collector current knowing the base voltage? The voltage is the result of current through the base, not the cause. When we pull current through a resistor a voltage develops across it. Why do you look at silicon any different? he voltage does not cause the current flow. Or does it?

Hmmm.

Okay, yes it is the voltage. We can have multiple secondary transistors. Each will, in theory, have the same collector current. The primary current does not change. Good point!
Where does this show up on a data sheet?

You can use Ebers moll to work out the collector current vs Vbe, IC=Is*e^(Vbe/Vt). Although Is and Vt are not normally on the data sheet; the infomation can be found. In most cases a close approximation could be made on these values and I don't see this any worse than the random values that people use for Vbe when calculating the collector current using the base current method.

Cheers
Adam

 

[Herschel Peeler]

You can use Ebers moll to work out the collector current vs Vbe, IC=Is*e^(Vbe/Vt). Although Is and Vt are not normally on the data sheet; the infomation can be found. In most cases a close approximation could be made on these values and I don't see this any worse than the random values that people use for Vbe when calculating the collector current using the base current method.

Cheers
Adam

Super! Thank you for the education. What convinced me was the current mirror argument. I couldn't explain it any other way. Ebers Moll? Not in my books.