Introduction to concepts in dc bias circuit design

Introduction to concepts in dc bias circuit design of the amplifier (BJT CE amplifier design-Voltage divider bias )

Outline • Concept of faithful amplification • Identification of dc circuits • What is Q point? • How to measure and locate Q point? • Interpret location of Q point

Concept of faithful amplification • The major goal of amplifier is to amplify applied input signal faithfully. • When is applied to the amplifier then different types of output waveforms are possible. Let us find out what are the possibilities

Faithful amplification-key specification • The waveforms shown below are possible outputs of the amplifier • Can you decide which of the above waveform represents faithful amplification of applied input signal( )? • (click on the waveform to select answer)

Look at upper part of waveform it is clipped which shows distortion in output waveform and output is not faithful amplification of input signal Try again

• Look at lower part of waveform , it is clipped which shows distortion in output waveform and output is not faithful amplification of input signal Try again

Look at both lower and upper part of waveform , it is clipped which shows distortion in output waveform and output is not faithful amplification of input signal Try again

• You are right! This is perfect sine wave with no clippings at extremities and known as faithful amplification Go back to question Next

Essential components to decide faithful amplification

For BJT amplifier circuit faithful amplification can be predicted from a)Amplitude of input signal and dc bias circuit b)Dc bias circuit and ac circuit c)Ac circuit and amplitude of input signal

• This combination of ac and dc circuit helps to decide only one parameter required to predict faithful amplification and thus not sufficient. Try again

• For this combination, amplitude of input signal is partially useful to decide faithful amplification but not complete specification and ac circuit alone is not sufficient , it needs dc circuit to decide other parameter required to predict faithful amplification. • Try again

• You are right ! • Faithful amplification of the amplifier can be predicted from dc bias circuit and input signal amplitude. Go back to question Next

Identification of dc circuits We will study effect of dc bias circuit on amplifier design Let us find what is dc bias circuit

• For the given CE amplifier circuit identify which is appropriate dc circuit? Select option by clicking on the circuit below Vcc

• In the selected circuit capacitors are present , but for dc condition you need to open circuit capacitors and only resistors in the circuit will remain. Try again

• You are right!. For dc condition all capacitors open circuited and only resistors in the circuit will remain. The circuit for dc conditions is Vcc Go back to question Next

Dc circuit parameter Let us find what are important parameters of dc circuit to decide faithful amplification

Q. Which of the following do you think is useful to predict faithful amplification? (click the box to select option) Value of βdc Resistor values Q point location

• The values are helpful in calculating current and voltage values of the dc circuit but are not useful to predict faithful amplification Try again

• You are right! Faithful amplification depends on location of Q point Go back to question Next

How to locate Q point? • To locate Q point which of the following characteristics are important? Ic= Vcc/Rc+ RE a)Output characteristics b)Load Line Vcc c)Input characteristics • (Select options by clicking the buttons) Only a only b only c a&b both a & c both b&c both

• You are right partially. One of the important characteristics to locate Q point is output characteristics , but not sufficient to locate Q point. Select other options

Input characteristics are useful to locate Q point partially (i. e. value of IBQ)but are not useful to find other values of Q point(VCEQ, ICQ ) Try again

• You are right partially. One of the important characteristics to locate Q point is load line , but not sufficient to locate Q point. Select other options

• This combination cannot fix Q point since , it is only set of characteristics. Try again

• Load line is useful and with input characteristics it can locate only IBQ value. Other values(VCEQ and ICQ can not be calculated. Try again

• You are right! you need both output characteristics and load line to locate Q point Go back to question Next

Animation below will show Q point is located IB 8 = Collector Current( IC) Ic= Vcc/Rc+ RE Output characteristics Lo ad lin e IBQ Q point • 1. To locate Q point we need to draw Output characteristics • Output characteristics –This is a set of characteristics. For each fixed value of base current(IB) the relation between collector to emitter voltage(VCE) and collector current(IC) is plotted. IB 7= IB 6= • 2. Superimpose load line on the output characteristics • Load Line—The line joining voltage Vcc and current Ic= Vcc/Rc+ RE IB 5 = IB 4 = IB 3 = IB 2 = IB 1 = 3. Mark IBQ characteristics IBQ—Base current of the given circuit Vcc under no signal condition. This is also known as dc bias current. IBQ characteristics are output characteristics for IBQ The point of intersection of IBQ characteristics and load line is “Qpoint” “Q point” is thus point on the load line representing dc bias conditions of the amplifier circuit

Collector Current( IC) Find VCEQ and ICQ IB 8 = Output characteristics IB 7= IB 6= IB 5 = IB 4 = Load line ICQ IBQ Q point Collector current under no signal condition also known as dc bias current(Q point current) VCEQ IB 3 = IB 2 = IB 1 = Collector to emitter voltage under no signal condition also known as dc bias voltage(Q point voltage)

How to measure Q point • To measure Q point, we measure value of base current , collector current and collector voltage under dc conditions(no Connect signal condition). voltmeter here to measure VCEQ Connect ammeter here to measure ICQ currents IBQ VCEQ

Locate Q point. Once we measure values of currents and voltage we can locate Q point by two methods In the first method we fix output characteristics for IBQ(measured value) then draw load line(by joining. VCC and Ic. ). Fix point as intersection of the two Collector Current( IC) IB 8 = Ic= Vcc/Rc+ RE(4. 08 m. A Lo ) ad IB 6= lin e Q point IBQ IB 7= IB 5 = IB 4 = IB 3 = IB 2 = IB 1 = Vcc=20 V

• In the second method we use VCEQ and ICQ values and load line as shown Collector Current( IC) Ic= Vcc/Rc+ RE=4. 08 m. A ICQ IB 8 = Lo ad lin e IB 7= IB 6= IB 5 = Q point IB 4 = IB 3 = IB 2 =IB 1 = VCEQ Vcc=20 V

Interpretation of Q point • Once we locate Q point by any of the two methods , we need to interpret the location of Q point. • First we will study different regions of load line which will help us in interpretation of Q point. • Remember load line is essential characteristics to locate and interpret Q point location.

Collector Current( IC) gion e r n ratio 40 µA Satu 4 m. A 30 µA 3 m. A 20 µA 2 m. A 10 µA 1 m. A In the output characteristics the region below(slashed) this point is cut off region(here both transistor junctions are reverse biased) and transistor is switched off VBE<0. 7 V Cut off region In the output characteristics the region beyond this point (slashed) is saturation region and transistor saturates(both junctions of transistor are forward biased). Output current is constant and there is no relation between input and output current VCE=0. 1 V This diagram explains Important regions of load line

Interpretation of Q point(contd) • In the process of interpretation of Q point • Second important aspect is how to draw output waveforms for applied input voltage for given location of Q point. • In the next slide Q point is already located and we will observe how can we draw output waveform?

Input voltage gion e r ion t a r u 40 µA Colle at S ctor 4 m. A Curre nt( IC) 3 m. A FIG. 1 30 µA Q 20 µA 2 m. A 1 m. A output current waveform 10 µA Cut off region output voltage waveform Apply input voltage

Why Q point location is important? • If we change location of Q point will the shape of output waveform change? • Let us vary location of Q point on the load line and observe the output waveform

Collector Current( IC) gion e r n ratio Satu 4 m. A 40 µA 3 m. A 20 µA 2 m. A 10 µA Change the location of Q point by selecting value of IBQ Click on the box to select value of IBQ 1 m. A Cut off region Go to design tip

Collector Current( IC) gion e r n ratio Satu 4 m. A 40 µA FIG. 1 30 µA 3 m. A Input voltage 20 µA 2 m. A 1 m. A Q 10 µA Cut off region output current waveform This is output waveform in which upper part of waveform is clipped , hence does not represent faithful amplification Q point is at lower extremity and input goes to cut off region) output voltage Base voltage< waveform 0. 7 V Vary Q point

Input voltage gion e r ion t a r u 40 µA Colle at S ctor 4 m. A Curre nt( IC) FIG. 1 30 µA 3 m. A 2 m. A 1 m. A output current waveform • This is perfect sine wave with no clippings at extremities and known as faithful amplification Q • Q point at the centre of load line 20 µA 10 µA Cut off region output voltage waveform Vary Q point

Input voltage Q gion output e r tion a r current u Sat waveform Collector Current( IC) 4 m. A Q point at upper extremity and upper portion of input voltage saturates transistor 40 µA FIG. 1 30 µA 3 m. A 20 µA 2 m. A 10 µA 1 m. A Cut off region VCEQ= 0. 1 V output voltage waveform This is output waveform in which only upper portion of the waveform is present and lower portion is clipped , hence does not represent faithful amplification Vary Q point

Collector Current( IC) gion e r ion t a r u Input voltage Sat 4 m. A 40 µA 3 m. A Q FIG. 1 2 m. A output current waveform 1 m. A For this location of Q point upper portion of the input just reaches to saturation 20 µA 10 µA Cut off region output voltage waveform This is output waveform in which small part of lower portion of the waveform is clipped , hence does not represent faithful amplification Vary Q point

Design tip 1 If Q point is located at the extremities then output waveforms are clipped at upper extremity or lower extremity. It is important to decide Q point location near centre to get faithful amplification Practical value of VCEQ= Supply(VCC)/2(approx. ) You can go back to find effect of Q point location

Faithful amplification-key specification Explanatory feedback Look at upper part of waveform it is clipped which shows distortion in output waveform and output is not faithful amplification of input signal