Some electronics questions

[ahmedragia21]

Hi i would like to read an answer for this questions

1. For a bjt whethere its used as an ampilifier or as a switch , does the Ic component is drawn from the supply depends on the current of the base multiplied by beta ? is this what Ic = B Ib means ? so a bjt just senses the Ib and output the amplified current ? but how the output is dc component with ac component from where is there the ac component ?

2. How in the FETs and MOSEFTs there's no Ig and there's a voltage on the gate , you will say the gate is insulated yea , so how the VG affects the FET to control the Idrain ?

3. If i wanted to have a constant 3.3V i would form a voltage divieder using two resistors .. but why we dont use it instead of voltage regulators ? maybe because resistors doesnt supply a constant current ? or because of the limited power disspation in resistors ?

Thanks

 

[ahmedragia21]

I couldnt find the thread in the forums !

 

[dknguyen]

2. You first need to know how a diode works- all the stuff about majority and minority carriers and depletion regions. The VGs of a MOSFET is insuated, but produces an electric field which attracts away charge carriers closer to the gate which make it easier for charge to flow- so it does. THat is how a MOSFET works in the simplest simplest sense.

3. The problem isn't constant current, but CONSTANT VOLTAGE that is INDEPENDENT OF THE LOAD CURRENT. The problem is due to both power dissipation and constant voltage combined together. A voltage divider ONLY outputs the voltage you calculate it to if no load current is being drawn (if the voltage divider is supplying a load, the load current is an extra current that flows through the top resistor, but not the bottom resistor so the voltage divider output sags below ideal). Therefore the output voltage of a divider is not independent of the current. TO make it behave as close to ideal as possible when supplying a load you need small resistors. BUt this means the divider burns a lot of current for no reason at all due to the current that is always flowing between the power rails through both divider resistors.

 

[ahmedragia21]

thanks , but would you please answer my first question ?

 

[crutschow]

Hi i would like to read an answer for this questions

1. For a bjt whethere its used as an ampilifier or as a switch , does the Ic component is drawn from the supply depends on the current of the base multiplied by beta ? is this what Ic = B Ib means ? so a bjt just senses the Ib and output the amplified current ? but how the output is dc component with ac component from where is there the ac component ?

2. How in the FETs and MOSEFTs there's no Ig and there's a voltage on the gate , you will say the gate is insulated yea , so how the VG affects the FET to control the Idrain ?

3. If i wanted to have a constant 3.3V i would form a voltage divieder using two resistors .. but why we dont use it instead of voltage regulators ? maybe because resistors doesnt supply a constant current ? or because of the limited power disspation in resistors ?

This may be somewhat duplicating dknguyen's comments but I had just finished writing when the post appeared. So here's my take.

1. Yes, the collector current is the base current times the current gain, beta. It does that same for both AC and DC currents. (But of course to amplify AC the transistor must be biased with a DC current greater than the AC to avoid distortion and clipping).

2. The operation of a FET is implied in its name (Field Effect Transistor). The gate voltage generates an electric field the controls the current flow in the source to drain channel. In a depletion-mode junction FET the current flows with zero gate voltage and a voltage is applied to pinch off the channel and slow the current flow (negative voltage for an N-Channel FET for example). An enhancement mode MOSFET is normally off and requires a gate voltage to attract carriers into the channel and start current flow (positive voltage for an N-Channel FET for example).

3. A resistive divider will give the calculated voltage only into a high impedance load (load much higher than the thevenin equivalent resistance). Thus to deliver a given voltage into a low or varying impedance load, a voltage regulators are used, which have very low output impedance.

 

[ahmedragia21]

thanks but for question 1 , you didnt tell me , does the transistor sense ib then it draws what it wants from the VCC and mul this by beta ?

 

[dknguyen]

Yes, Ic ~= Ibe * Beta. THat's simple enough...if beta was always the same. But in reality, it is not. It varies with Ib and Ic and is given in datasheets as a graph.

Because of this, it becomes easier to deal with BJTs by splitting things up into AC and DC. For example, for situations where the AC signal is "small" (which is a relative term meaning that across the entire AC signal range, beta does not change enough to make a difference for your application), you calculate the DC bias required to get this "approximate beta" for the AC signal that you need. YOu then bias the signal at that level to get that beta, which you assume will be the same for the entire range of AC signal you expect and you DC-block filters and such to recover the AC signal after the BJT has done the amplification.

 

[ahmedragia21]

so a transistor can be thought of as a current sense current ampilifier ?

 

[dknguyen]

so a BJT transistor can be thought of as a current sense current ampilifier ?

For a BJT, yes (not a MOSFET) - but not a perfect one since the gain changes with the input. For "pencil and paper" analysis for BJTs, we break the BJTs down into other circuit components which include a current-dependent current source.

MOSFETs are more like voltage-controlled variable resistors where the relationship between GATE-SOURCE voltage and SOURCE-DRAIN resistance is NOT constant.

Different circuit models approximate BJT behaviour. Here is one example out of many:
Image:H pi model.png - Wikipedia, the free encyclopedia

THere are also circuit models breaking down MOSFETs into simpler circuit components.

 

[ahmedragia21]

But is a mosfet as a variable resistor acts like a BJT ?