Basic Amplifier designing helps to learn how to design Single Stage Amplifiers.

Adjustment of quiscent point is not discussed.

E-mail: mashood.ahmad32@gmail.com

HOW TO DESIGN A PRACTICAL AMPLIFIER

Designing means selecting or making a circuit.

It also includes finding unknown elements in a circuit.

DESIGN PROBLEM

To make an amplifier with a voltage gain

AV=____ using single stage BJT amplifiers

SELECTION

 Quantity

Common Emitter (CE)

Common Collector (CC)

Common Base (CB)

AI Current Gain High High Low (Unity)

AV Voltage Gain High Low (Unity) High

Ri Input Resistance Medium High Low

Ro Out Resistance High Low High

CHOSEN CONFIGURATION Common Emitter Amplifier

MODIFICATION IN CHOSEN CONFIGURATION

Reasons for modification:Rsig<< RB

Stable biasingIt doesn’t have an actual current sourceDecreases gain & not needed in test circuit

Back to Slide 10

Formulas for Common Emitter Amplifier

Back to Slide 10

START ACTUAL DESIGNING

Where do we start from then?• Since we are designing the

amplifier, we need to make sure that circuit is operating in active mode. i.e. The quiescent point is fine.

QUIESCENT POINT

FINDING UNKNOWN ELEMENTS

+

-

VCE(Q)

+

-

VE=IERE

+

-

ICRC=VCC-VC

We know VCE(Q). We know AVVT=ICRC. We can find VE=IERE. We can find VB=VE+VBE.• If we know IC, we can find IE & IB b/c we know β.• If we know IC &IE, we can find RC & RE.• If we know current through RB1 we can find RB1 and thus RB2.• Current through RB1 is in range of IE to 0.1IE. Better near 0.1IE.

A DIFFERENT APPROACH We started from quiescent point and

although we found all the unknowns, we still overlooked some factors that may destabilize the design.

Slide 5 & Slide 6 In addition to these we didn’t knew the input signal swing. So, we use a little different approach to counter these problems.

OVERLOOKED FACTORS First of all, we brought RB1, RB2 & RE in without much argument. But, actually they brought two more conditions to be met; to make IE insensitive to β & temperature variations. This puts an ultimate limit to signal swing (VCE(Q)) & gain (ICRC).

OVERLOOKED FACTORS REASONING

1. Making VBB much higher will make VE much higher thus lowering the VCE(Q) & ICRC giving less signal swing & gain. But we cannot escape this.2. We can use lower value of RB1 & RB2 but then RB will be less and will approach Rsig.

OVERLOOKED FACTORS CORRECTION

1. For signal swing, gain & large VBB, a compromise is proposed. ICRC=VCB(or VCE) = VBB=VCC/32. For large RB, we keep current through the divider in range of IE to 0.1IE. Better if it is near 0.1IE.

FINDING UNKNOWN ELEMENTS

+

-

VCE(Q)

+

-

VE=IERE

+

-

ICRC=VCC-VC

We know VCE(Q)=VCC/3. We know AVVT=ICRC=VCC/3. We can find VE=IERE. We can find VB=VE+VBE.• If we know IC, we can find IE & IB b/c we know β.• If we know IC &IE, we can find RC & RE.• If we know current through RB1 we can find RB1 and thus RB2.

FINDING RB1 & RB2VCC

gnd

0.1 IE VB

LIMIT ON GAIN1. If we keep ICRC=VCC/3, we will always have a constant gain.2. So, we can always change our ICRC a little less or more than VCC/3 and VCB & VBB(VB) can be adjusted accordingly.3. There will always be a trade-off between gain(ICRC) & signal swing(VCE).

FINDING ACTUAL β PRACTICALLY

CAPACITORS

FINAL REMARKS

We kept on assuming IC but never actually talked about what its value can be. If we don’t know how to set IC (IE & IB also depends upon it), our design is still nothing.

Fortunately any value of IC less than IC(MAX) given in the datasheet can be used as long as PC (Power) does not exceeds.

Typically IC can be chosen 1-10 mA. Input Voltage swing is also an important factor.

REFRENCES

Slide #2 : A/C Ajmal Jauhar Analog E-II Slideshow titled “Lec 01 Introduction”

Most of the other figures are from Microelectronic Circuits 5/e Chapter#5