Different use of transistor biasing methods

transistor bias

hi there, please help me with this question, there is many method to bias the transistor, for example voltage divider bias, collector feedback bias, emitter bias, base bias, etc, please tell me when should we use the most appropriate of these type of biasing the transistor?

and then what is the different between independence source and dependence source? what is the example of current source?

thank you very much

Re: transistor bias

Hi zhi_yi,

I hope the following helps.

Voltage Divider Bias
- commonly used for biasing BJT single transistor amplifier, especially in a common-emitter configuration to provide for medium current gain and medium current gain.
- bias the base voltage so that you have a DC level for the input signal (AC) to swing.
- the resistors are selected to provide sufficiently small DC current to flow into the base. R1/R2 ratio is usually between 0.5 to 0.75. R2 is usually bigger because (1) you don't wish the circuit to pick up noise from the ground, (2) emitter has another resistor connected to it to reduce static power consumption, thus level up the emitter voltage, base voltage has to level up as well, (3) you don't want too much DC current flowing down to ground.

Collector Feedback Bias
- Commonly used in Common-Base Configuration, especially in Differential Amplifier of Analog and RF circuits.
- Collector Feedback eliminates Miller Effect due to Miller Capacitance between base and collector by surpressing it with transconductance gm. This offers stability (I shall not show you the mathematics because it is analog IC level with some feedback control principles which you might not need to know now)
- This is necessary only for very high frequency amplification because signal from base can couple from base to collector even though base-collector junction is reversed biased (thus a capacitor)and a capacitor decreases in reactance as frequency increases because of Xc = 1/jwC where w = 2 PI f.

Emitter Bias and Base Bias are not very useful these days. You might want to read Sedra & Smith "Microelectronic Circuits" in details. Either impedance is too low or the gain is too little.

Independent sources are implementations of ideal sources which does not change with variation in output impedance. Examples are constant current and constant sources. Controlled sources such as current mirrors are also independent sources, despite they are controlled.

Dependent sources are sources that vary with load impedance. Such are voltage divider output and current divider output. Changing the resistance varies the source magnitude.

Example of a current source is the MOS transistor that is operating in saturation region. Varying the gate-source voltage Vgs varies the drain current Id. MOS transistor operating in this way is usually called a voltage-controlled current-source by analog IC designers.

Re: transistor bias

waa.. thank you very much..

but i still a little confused, why the noise will increased if we reduce the R2? it said R2 should be bigger so the noise can't be pickked up from the ground and amplified? is it the noise will go to the ground? and if the resistance of R2 is not high enough, so the noise will flow into the base and amplified?

(2) emitter has another resistor connected to it to reduce static power consumption, thus level up the emitter voltage, base voltage has to level up as well <-- what is static power consumption?

thank you very much........

Re: transistor bias

Yes, correct. You understood. Thermal=Johnson=White Noise is dominant in any conductor. Ground in principle is clean but practically it is a conductor too, thus have white noise. Sometimes highly-frequency noice can be picked by the ground since it is a conductor, R2 impedes this high-frequency or stray noise. Thus R2 ensures that these noises do not get injected into the base hence prevented amplification of the noises.

static power consumption occurs when the amplifier is always consuming power whether or not the amplifier is amplifying any signal. for example, if the input signal is not injected into the base, the amplifier still consumes power and this is called static power consumption. The British has a very elegant word, called the quiescent power consumption, the same meaning.

static power consumption is determined by the value of Re since Ve/Re = Ie or Icq that sets the quiescent operating current. however the condition for voltage divider bias stands only if Vb = Ve + Vbe for the amplifier to operating. therefore if Ve inscreases, Vb must also increase accordingly.

Re: transistor bias

okay, thank you

what is thermal johnson white noise???

what is stray noise?? is it the noise occured in the high frequency because of the internal capacitance of the transistor (miller effect) ???

Sometimes highly-frequency noice can be picked by the ground since it is a conductor, <- so the noise must can be picked up from the Vin too, right? from Ib, because Ib flow to base of transistor, and it's flow by conductor too, so the noise can be come from anywhere in high frequency, is it right? please correct me

what makes the common base can work in the high frequency? the common base can work better in high frequency is it mean that the common base can filter the noise? or is it mean that the common base gain doesn't decreased in high frequency?

oo.. okay.. so it is the meaning of static power consumption..

thank you very much

Re: transistor bias

You are welcome.

Thermal noise, also known as Johnson Noise or White Noise. Scientifically called thermal noise because it is the noise caused by the random motions of the charged carriers in the conductor. Someone by the name Johnson discovered this phenomenon and this is named after him - Johnson Noise. The characteristic of thermal noise is such that it spreads over the entire EM spectrum, giving an average or uniform floor of about 300 to 400 mV. Some vision and colour spectrum scientists discovered that thermal noise produces white colour because white noise spreads over the entire EM spectrum, hence the entire visibile colour spectrum as well, thus combining all colours or all colour frequencies yield white noise.

Stray noise is any noise not generated by the system or injected by the signal noise, in other words, noise picked up from outside the signal source and system. Stay noise can be cosmic radiation, EMI from mobile and wireless interference, 60 Hz power mains, etc.

Miller Effect is within the device. Not a stray noise.

We design the system and circuit to work in principle, considering sources and devices. However the actual circuit has many signal-carrying conductors that couple crosstalks to one another. Your signal source to base can couple to output if placed close together, especially when the signal source is high frequency in nature, such as a clock. So isolation of signals and noises depend on how you connect and arrange your circuit topologies.

Common-base demonstrate high-pass frequency response, thus it passes high frequencies, not filter them.

I hope all these information helps.

Re: transistor bias

ooo.. thank you

Miller Effect is within the device. Not a stray noise. <-- so, what is the miller effect?

i want to design an amplifier, but i am confused to choose the appropriate transistors, which transistor should i choose if i want to design for example class A, or class AB, etc, should i read the datasheet first, and find out the maximum breakdown voltage? and then i determine which one i have to use in my circuit? please help me to distinguish it


Common-base demonstrate high-pass frequency response, thus it passes high frequencies, not filter them <-- which part of the common base demostrate the high pass frequency? is it the capasitor that series with the load? (the couplling capasitor), because in the high frequency the capasitive resistancy will decreased, so almost all of the amplified signal will go to the load? but it is the same in the common emitter, why the common emitter can't work better in high frequency as like as in the common base configuration? please help me

what is the advantage of using JFETs?

thank you very much

Re: transistor bias

Well... Since I can't possibly write a long essay to give you full explanation to what you hope to understand. I can only answer your questions in brief.

Miller effect is a multiplication effect of the capacitance Cµ (due to reversed biased junction diode base-collector of the Bipolar transistor) by a large negative gain (-gm.R'load), where gm is the intrinsic gain and R'load is the small-signal load resistance.

It is Miller effect of Cµ and Cπ (between base-emitter) that limits the upper cutoff frequency of the common-emitter amplifier. In other words, common-emitter exhibits a low-pass characteristic.

To design a single-transistor amplifier circuit, you need to consider many factors. Here are some pointers:
1. Bandwidth, lower and upper cutoff frequencies
2. Gain, feedback, stability
3. DC biasing (Class A, AB, etc)
4. Voltage supply rail (12, 9, 5V etc)

The above factors, plus physical size if considered, decides the choice of transistors to use.
There are many monolithic BJT discretely packaged for sales. You can find many when you flip Farnell catelog, Rapids, RS and Maplins etc.

In short, the absence of Miller effect in common-base gives it a high-pass charactertics.

JFET is easier to fabricate than BJT by using lateral process. JFET has better linearity than BJT.

If you wish to know more, you can read this popular undegraduate textbook written by
Sedra & Smith, "Microelectronic Circuits 5th Ed."

Of course, there are many good books too.

I hope this information in brief helps. Cheers!

Re: transistor bias

okay skyhigh.. thank you...

can i download the book from internet (free )??

thank you

Re: transistor bias

Sorry, I don't have an electronic copy of the book. I am using a printed book.

If you have eMule P2P software, you might be able to search from others who are sharing the soft copy.

Re: transistor bias

oo... okay.. thank you