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Op-amp in closed loop configuration

arjun — Wed, 15 Oct 2008 17:12:58 +0000

Op-amp is in closed loop configuration when there is a connection either direct or via another network exist between the input and output terminals.We can control the gain of op-amp if we introduce a modification in the basic circuit.This modification involves a feedback ie the input signal is fed back to the input either direct or via another network. Different feedback configurations An op-amp that uses feedback is called a feedback amplifier.A feedback amplifier is sometimes referred to as a closed loop amplifier because the feedback forms a closed loop between the input and the output.A close loop amplifier can be represented by using two blocks one for op-amp and the other for a feedback circuit.These connections are classified according to whether the voltage or current is fed read more

Open Loop Op-Amp Configurations

arjun — Tue, 14 Oct 2008 17:35:05 +0000

When connected in open-loop configuration,the op-amp functions as a high gain amplifier.These are three open loop op-amp configurations. Differential Amplifier Inverting Amplifier Non-Inverting Amplifier These configurations are classed according to the number of inputs used and the terminal to which the input is applied when a single input is used. Disadvantages of open loop configurations The open loop of the op-amp is very high.Therefore only the smaller signals having low frequency may be amplified accurately without distortion. Open loop Voltage gain of the op-amp is not a constant voltage gain varies with changes in temperature and power supply as well as mass production techniques.This makes op-amp unsuitable for many linear applications Bandwidth of most open loop op-amps is read more

Ideal Voltage Transfer Curve

arjun — Tue, 14 Oct 2008 15:59:13 +0000

Vo=A(V1-V2) This is the basic op-amp equation in which the output offset voltage is assumed to be zero.The graphic representation of this equation is shown;where the output voltage ,Vo is plotted against input difference voltage Vid,keeping gain A constant.The output voltage cannot exceed the positive and negative saturation voltage.These saturation voltages are specified by an output voltage swing ratings of an op-amp for given values of supply voltages.The output voltage is directly proportional to the input difference voltage until it reaches the saturation voltages and thereafter the output voltage remains constant. This curve is called ideal voltage transfer curve Share and Enjoy: read more

The Ideal Op-amp

arjun — Mon, 13 Oct 2008 15:36:21 +0000

An ideal op-amp would exhibit the following characteristics Infinite Voltage gain,A Infinite input resistance,Ri so that almost any signal source can drive it and there is no loading of the preceding stage Zero output resistance,Ro so that output can drive infinite number of other devices Zero output voltage when input voltage is zero Infinite bandwidth so that any frequency signal from zero to infinite Hertz can be amplified without attenuation Infinite common mode rejection ratio(CMRR)so that the output common mode noise voltage is zero Infinite slew rate so that the output voltage changes occur simultaneously with input voltage changes Equivalent Circuit of an Op-Amp Output volatge,Vo=AVid=A(V1-V2) Vid=difference input voltage V1= Voltage at the non-inverting input terminal with read more

Op-amp Parameters

arjun — Mon, 13 Oct 2008 14:39:59 +0000

1)Input offset Voltage Input offset voltage is the the voltage that must be applied between the two input terminals of an op-amp to null the output.Typical value of 741 IC is 6mV dc. 2) Input offset current The algebraic difference between the current in the inverting and non inverting terminal is known as the input offset current Iio.As the matching between two terminals increases,the difference between IB1 and IB2 become smaller.Typical value for 741 IC is 200mA(max). 3)Input Bias Current IB is the average current flows in the inverting and non-inverting terminal of an op-amp. IB = (IB1 + IB2 )/2 Typical value for 741 is 500mA 4) Large Signal Voltage gain It is the ratio of the output voltage and the differential input voltage A = Output voltage/Differential input = Vo/Vid Typical read more

Frequency response of an op-amp

arjun — Sat, 27 Sep 2008 15:03:23 +0000

We have treated the gain of the op-amp as a constant.However it is a complex number that is a function of frequency.Therefore at a given frequency the gain will have a specific magnitude as well as phase angle.The manner in which the gain of the op-amp response to different frequencies is called the frequency response.A graph of the magnitude of the gain and frequency is called a frequency plot. Share and Enjoy: read more

Inverting and Non-inverting Amplifier

arjun — Sat, 27 Sep 2008 08:58:20 +0000

In the inverting amplifier,only one input is applied and that is to the inverting input terminal.The non-inverting input terminal is grounded since V1=0V and V2=Vin. Therefore Vo = -A(Vin) The negative sign indicates the output voltage is out of phase with respect to input by 180 degree or is of opposite polarity.Thus in the inverting amplifier,The input signal is amplified by gain A and is also inverted at the output. Non-Inverting Amplifier In this configuration ,the input is given to the non inverting input terminal and the inverting terminal is connected to ground. In the circuit,V1=Vin and V2=0V therefore the output voltage Vo=AVin.This means that the output voltage is larger than the input voltage by gain A and is in phase with the input signal Share and read more

Differential Amplifier

arjun — Sat, 27 Sep 2008 06:46:36 +0000

The figure shows the open loop configuration of differential amplifier in which the input signals Vin1 and Vin2 are applied to the positive and negative input terminals since the op-amp amplifies the difference between the two input signals,this configuration is called differential amplifier.Vin1 and Vin2 could be either ac or dc voltages.The source resistances Rin1 and Rin2 are negligible compared to the input resistance Ri.The voltage drop across the resistors can be assumed to be zero,which then implies that V1=Vin and V2=Vin2.The the output voltage Vo=A(Vin1-Vin2). The output voltage is equal to the voltage gain A times the difference between the two input voltage.The polarity of the output voltage is dependent on the polarity of the input difference voltage(Vin1-Vin2).In open loop read more

Output Stage of an op-amp

arjun — Fri, 26 Sep 2008 18:21:05 +0000

The function of the last stage in an op-amp is to supply the load current and provide a low impedance output.a simple output stage is an emitter follower with complementary transistors.When Vi is positive,transistor Q1 is on and supplies current to load RL .If Vi is negative,Q1 is cutoff and Q2 acts as a sink to remove current from the load RL .There is however,a limitation in this circuit.The output voltage VO remains zero volt until the input Viexceeds VBE(cut in)=0.5V.This is called cross over distortion.It can be eliminated by applying a bias voltage V greater than QVBE(cut in)=1V between the two bases so that a small current flows in the transistor even in the quiescent state. Share and Enjoy: read more

Level Translator

arjun — Fri, 26 Sep 2008 18:16:24 +0000

There are two good reasons for using a level shifter in an IC(op-amp).As we want an op-amp to operate down to dc,no coupling capacitor is used.Because of direct coupling,the dc level rises from stage to stage.The increase in dc level tend to shift the operating point of next stage.This,in turn,limits the output voltage swing and even distort the output signal.It,therefore becomes essential to shift the quiescent voltage before it is applied to the next stage.Another requirement to be satisfied is that the output should have voltage level of 0V for input signal. he simplest type of a level shifter is shown in fig.1 given below.This is basically an emitter follower.Hence the level shifter also acts as buffer to isolate the high gain stages from output stage.The amount of shift read more