## Frequency compensation

For low closed loop gain and high band width frequency compensation is used in op-amp There are two types of frequency compensation techniques they are:Internal frequency compensation technique.External frequency compensation technique.Internal Frequency Compensation Technique :It makes use of internal components for compensation.Usually used in instrumentation amplifiers. Also called as internally compensated op-amp.IC741 contains capacitance C_1 = 30pf which internally shunts off signal current and thus reduces available out put signal at higher frequency.The internal capacitance C1= 30pf the compensating  element which causes open loop-gain to roll off at -20dB/decade rate and gives a stable circuit.Bandwidth product of op-amp is 1mHz and if IC741 is wired for a closed loop gain of 104/80dB then its bandwidth is 100hz for a gain of 102 bandwidth.  The bandwidth increase to 10KHz with gain one making bandwidth 1Mhz. External Frequency Compensation Technique :These circuit fuse external frequency components to give low closed loop gain. The two common methods are:Dominant pole compensation.Pole zero compensation.  ...

## Stability of An Op-Amp:

Consider an op-amp with negative feedback which uses a feedback resistor network which may be used as an inverting amplifier for V­­2 = 0 and non-inverting amplifier V1 = 0 The closed loop transfer function is given by ACL = A/1+Aβ Where; A : open loop voltage gain                B : Feedback ratio If (1+Aβ) = 0 the circuit will become unstable and gives sustained oscillations. Re-writing 1+Aβ = 0 as 1-(-Aβ) = 0 leads to –Aβ = 1 Aβ is a complex quality Therefore, │Aβ│ = 1 and phase condition Lle- Aβ =0 or (multiple of 2π)                                                                           Lle- Aβ =1 or (multiple of π)   Since, resistor is the present in feedback network it does't provide any phase shift. When op-amp is used in inverting mode it provides phase shift  Of 180o and at low frequencies. However at high frequencies due to each corner frequency an additional phase shift of maximum -90o can take place in open loop gain phase. So, for two corner frequencies maximum phase shift is added Hence at...

## AC Characteristics of an op-amp

For small signal sinusoidal applications the AC characteristics of op-amp are discussed as follows:Frequency responseStability of an op-ampFrequency CompensationSlew RateFrequency Response :An ideal op-amp should have infinite band width. If open loop gain is 90 dB with DC signal then it should remain the same for audio and radio frequencies also but practically the op-amp gain deceases / roll's off at higher frequencies hence a capacitor component is placed in a circuit of op-amp      For an op-amp with one corner frequency all the capacitor effects can be represented by single capacitor.     The following is a high frequency model of an op-amp:  The open loop voltage gain is obtained as:        V0  = [-jXc/(R0 - jXc)] AOLVd(or) A = V0/Vd = AOL/ 1+j2πfR0­CA = AOL/ 1+j(f/f1)f1 = 2πR0­CThe magnitude and phase angle of open loop gain is│A│ = AOL/ 1+(f/f1)2   (or) Ф = tan-1 (f/f1)...

## Thermal Drift

Bias Current, Offset current, offset voltage change with temperature then a circuit is maintained at 250 room temperature. It does not remain so, and temperature raises to 350 which is called as drift. Usually current drift is expressed in nA/0c where offset voltage drift is expressed in mv/0c.Effect of thermal drift is minimized by cooling or placing PCV away from heat           ...

## Input offset voltage

Inspite of all the compensation techniques the output voltage is still not zero with zero input voltage.  Hence a small voltage is applied at the input terminals to make output voltage zero which is called as input offset voltage. The following are the non-inverting and inverting amplifiers respectively: Where  V1 =0 gives Voltage V2 is given by    V2 = V0 [R1/(R1+ Rf)] Or  V0 = V2 [ (R1+ Rf)/ R1] Since, Vios = │V1 – V2│and Vi = 0 Vios = │0 – V2│ =  V2 V0 = [1+(Rf/R1)] Vios...

## Input Offset Current

It is the difference between the current flowing from inverting and non-inverting currents of an op-amp.              Ios = IB+ - IB-   With bias current compensation the offset current will produce an output voltage when the input voltage is zero We have,           V1 =IB+ Rcomp             And   I1 = V1/R1 KCL at node ‘a’ gives            I2 = (IB- - I1)               = IB- [IB+ (Rcomp/R1)] Also,                 V0 = I2 Rf - V1           V0 = I2 Rf - IB+ Rcomp              = [IB- - IB+Rcomp]Rf - IB+Rcomp We know, Rcomp = R1Rf / R1+Rf Substituting Rcomp, we have,         V0 = Rf [IB- - IB+] The effect of offset current can be minimized by keeping feedback resistance value be small  ...

## Characteristics of Op-amp

The non- ideal characteristics of op-amp which are DC and AC are:DC error components are:input bias current input offset current input offset voltage thermal driftInput Bias Current:The input bias current of an operational amplifier is the average of the two currents flowing at the inverting and non-inverting terminal respectively. The mathematical equation is given by:         IB = (IB++IB-)/2 For a Basic inverting op-amp : basic inverting op-amp If V1 = 0, then output voltage is offset given by,                V0 = IB - Rf1For 741 op-amp, Rf  = 1mΩ and input bias current is 500mA or less.Therefore, output voltage is driven to 500mV.In applications where signal levels are measure in milli-volts it is totally unacceptable. Hence, a compensated resistor is added between non-inverting input terminal and ground. Current IB+ going through the compensator resistor has a voltage V1 across itFrom KVL,-V1 + 0 + V2 – V0 = 0By selecting proper value of Rcomp V2 and be cancelled with V1 to make V0 Therefore, V1...

## Conditions of an Ideal Operational Amplifier

An op-amp is said to be ideal if it has the following characteristics. Open loop voltage gain (AOL) is infinity. Input resistance (Ri) is infinity. Output resistance (Ro) is zero. Bandwidth (BW) is infinity. Zero off-set (i.e) V0 = 0, when V1 = 0 & V2 = 0. An ideal op-amp cross no current and both the input terminal i 1 = i2 = 0. Because gain is infinite the voltage between inverting and non-inverting terminals i.e op-amp is essentially zero for infinite output voltage V0 is independent of the current drawn from the output is R0 = 0....