# Connexions

You are here: Home » Content » AnalogElectronics_Lecture4_PartC_RC coupled Amplifier Design Procedure.

### Recently Viewed

This feature requires Javascript to be enabled.

# AnalogElectronics_Lecture4_PartC_RC coupled Amplifier Design Procedure.

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: Part C of Lecture 4 gives the design procedure of RC coupled Amplifier for maximum symmetrical swing and for maximum voltage gain.

AnalogElectronics_Lecture4_PartC_RC coupled Amplifier Design Procedure.

CE RC COUPLED AMPLIFIER

DC equivalent circuit ;

We draw the thevenin equivalent circuit for the input port 1-1'

VB=Thevenin Equivalent voltage at 1-1'

Output mesh equation

Under this condition 100% stability of Q point is achieved with respect to aging, replacement and temperature.

In the expression

ICF(VB- VBE)/[RB+(1+βF)RE],

βF and VBE are temperature sensitive as well as BC reverse biased junction leakage current ICEO is also temperature sensitive.

Here βF(T)= βF(300K)2(T-300)/80,

ICEO(T)= ICEO(300)2(T-300)/10 ;

All these three temperature dependences are causes of Q point instability and thermal runaway. So if DC operating point is independent of these three terms we achieve 100% stability and prevent thermal runaway.

But practically it is impossible to meet the two conditions given in Eq.A .

Generally IC is taken in the range of 1mA to get the best performance out of the BJT in terms of βF and transit frequency ωT .

RB= (1/5) (1+βF)RE and VB = 3V. Eq. B

This gives a satisfactory stability of Q point and prevents thermal runaway. In self-bias, there is current-series feedback which is responsible for Q point stability.

Since we get optimum performance at 0.5mA < IC < 5mA, therefore we choose IC = 1mA.

For maximum symmetrical swing we take the voltage across RC = VCE = 5 V and we take voltage across RE =VRe = 2V for a decent stability.

In Figure 22, it is shown how distribution of voltage as shown in Figure 21 ensures maximum symmetrical swing.

Under static condition or under drift condition Q point traces the locus along static loadline and under signal condition it traces the dynamic loadline.

This gives RC = 5kΩ and RE = 2kΩ

Since Eq.B demands

;

Therefore RB= 40kΩ if βF = 100;

R1R2/( R1+R2)= RB=40k Eq.1.

VB= IBRB + 0.7V + 2V and IB= 1mA/100;

Therefore VB= 3.1V= VCCR2/( R1+R2) Eq.2

We have two simultaneous equations and two unknowns.

Solving them we get R1= 154k and R2= 54k;

The internal voltage gain

= vout/vin= -gmRC=-192.3;

Here we have not included the source resistance.

The input resistance=

Rin= rx+rπ= 100Ω+ 2.6kΩ= 2.7kΩ;

So here including RS will reduce the Voltage gain only slightly since RS may either be 50Ω or 600Ω which is negligible compared to 2.7kΩ.

The Band Width calculation will be seen in next section.

In CB BJT, input impedance is :

Rin = re= 26Ω for IE= 1mA.

Here RS = 50Ω or 600Ω is going to considerably reduce the Voltage Gain if RS is included while measuring the voltage gain. In this case internal voltage gain is going to be significantly higher.

What is difference between hybrid-parameters, hybrid-pi parameters and T-Model?

Hybrid Model is a mathematical model and abstract model whereas Hybrid-π or T Model are physical models. In data sheet both the parameters are given. We may work in terms of either model.

## Content actions

PDF | EPUB (?)

### What is an EPUB file?

EPUB is an electronic book format that can be read on a variety of mobile devices.

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

#### Definition of a lens

##### Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

##### What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

##### Who can create a lens?

Any individual member, a community, or a respected organization.

##### What are tags?

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks