RC Coupled Amplifier- Frequency Response 2022

RC Coupled Amplifier 

Rc cou­pled ampli­fi­er is a two stage CE ampli­fi­er using NDN tran­sis­tor is shown in the above pic­ture. It is a most pop­u­lar type of cou­pling because it is cheap and pro­vide excel­lent operation

Rc (Rc1 and R2) is the col­lec­tor load resis­tor. R1,R2 and Rg pro­vide bias­ing and sta­bi­liza­tion. Ce pre­vents the loss of ampli­fi­ca­tion due to neg­a­tive feedback. 

The capac­i­tors Ci and Co are used to block the DC sig­nal and trans­mit only AC sig­nal. The capac­i­tor Cc is used to con­nect the out­put (col­lec­tor) of first stage to the input (base) of sec­ond stage.

When an AC sig­nal is applied to the base of the first tran­sis­tor (Q1) it appears in the ampli­fied form across its col­lec­tor load reg­is­ter Rc1. This ampli­fied sig­nal is giv­en to the base of next stage through pling capac­i­tor Cc.. 

The sec­ond stage does fur­ther ampli­fi­ca­tion of the sig­nal. The first stage pro­duces 180° phase shift and the sec­ond stage pro­duces anoth­er 180° phase shift. So the total phase shift is 360°. Hence the out­put sig­nal is in phase with the applied input signal.

The over­all gain of this ampli­fi­er is equal to the prod­uct of the gain of two stages. The influ­ence of the capac­i­tors used in this cir­cuit, reduces the over­all gain of the ampli­fi­er at low and high fre­quen­cy ranges. 

The gain is high and con­stant, only at mid fre­quen­cy ranges. 

Frequency response

An ampli­fi­er should oper­ate irre­spec­tive of the fre­quen­cy of the input sig­nal. but prac­ti­cal­ly an ampli­fi­er does not mag­ni­fy all fre­quen­cy range of input sig­nal with an equal amount

Prac­ti­cal­ly, by using the reac­tive com­po­nents, the gain will be decreased at low and high fre­quen­cy ranges and con­stant only at mid fre­quen­cy ranges. This behav­iour of the ampli­fi­er is briefly explained below. 

1.Low Frequency Range 

‘At low fre­quen­cies, the reac­tance of cou­pling capac­i­tor ce is quit high and hence very low amount of sig­nal will pass from first stage to the sec­ond stage. So gain will be reduced. Sim­i­lar­ly the reac­tance of the bypass­ing capac­i­tor C is very high, hence it can­not shunt the emit­ter resis­tor R effec­tive­ly.
These two fac­tors reduce the volt­age gain of the ampli­fi­er at low fre­quen­cy range. 

High Frequency Range

At high fre­quen­cies the reac­tance of the cou­pling capac­i­tor is very low. Hence it behaves as a short cir­cuit. This increas­es the load­ing effect of next stage.
Sim­i­lar­ly at high fre­quen­cies the reac­tance of bypass­ing capac­i­tor Ce is very low which increas­es the base cur­rent. This reduces the cur­rent ampli­fi­ca­tion fac­tor B. These two fac­tors reduce the volt­age gain of the ampli­fi­er at high fre­quen­cy range. 

Mid Frequency Range

At mid fre­quen­cies the volt­age gain of the ampli­fi­er is con­stant. At this range the effect of cou­pling capac­i­tor is to main­tain a uni­form volt­age gain.
In this range as the fre­quen­cy increas­es, the reac­tance of C, decreas­es which tends to increase the volt­age gain. 

How­ev­er at the same time, low­er reac­tance means high­er load­ing of the first stage tends to decrease the Volt­age Gain these two fac­tors almost can­cel each oth­er result­ing in a uni­form gain at mid fre­quen­cy range 

RC Coupled Amplifier Frequency Response

The fre­quen­cy response char­ac­ter­is­tics of a two stage RC cou­pled ampli­fi­er is shown in the below picture 

The gain is reduced at low and high fre­quen­cy ranges and con­stant only at mid fre­quen­cy range. From the response curve we will cal­cu­late low­er cut off fre­quen­cy (fL),upper cut off fre­quen­cy (fu) and Band­width (BW)

Lower Cut Off Frequency 

The fre­quen­cy at which the volt­age gain of the ampli­fi­er is 70.7% of max­i­mum gain on low­er side of the fre­quen­cy range is called low­er cut off frequency 

Upper Cut Off Frequency 

The fre­quen­cy at which the volt­age gain of the ampli­fi­er is 70.7% of max­i­mum gain on the high­er side of the fre­quen­cy range is called upper cut off frequency 

Bandwidth

The range of fre­quen­cy over which the gain is equal to or greater than 70.7% of max­i­mum gain is known as bandwidth 

It is also defined as the fre­quen­cy dif­fer­ence between the upper cut off fre­quen­cy and low­er cut off frequency 

Advantages Of RC Coupled Amplifier

• Fre­quen­cy response is excellent 
• Cost is low 
• Non lin­ear dis­tor­tion is low 
• It is com­pact, light and small 

Disadvantages Of RC Coupled Amplifier

• Low volt­age gain and pow­er gain 
• Noise is pro­duced with age, par­tic­u­lar­ly in moist climates 
• Imped­ance match­ing is poor 

Also read: Work­ing Prin­ci­ple Of [PLC]Programmable Log­ic Con­troller And Best Features