Operational Amplifier ( OPAMP )

The OPAMP:

The op amp is one of the basic building blocks of linear design. In its classic form it
consists of two input terminals, one of which inverts the phase of the signal, the other preserves the phase, and an output terminal. the standard symbol shown in the picture above.

The name “op amp” is the standard abbreviation for operational amplifier. This name
comes from the early days of amplifier design, when the op amp was used in analog computers. (Yes, the first computers were analog in nature, rather than digital). When the basic amplifier was used with a few external components, various mathematical “operations” could be performed. One of the primary uses of analog computers was during WWII, when they were used for plotting ordinance trajectories.

The classic model of the voltage feedback op amp incorporates the following
characteristics:

- Infinite input impedance

- Infinite bandwidth

- Infinite gain

- Zero output impedance

- Zero power consumption

None of these can be actually realized, of course. How close we come to these ideals
determines the quality of the op amp. This is referred to as the voltage feedback model. This type of op amp comprises nearly all op amps below 10 MHz bandwidth and on the order of 90% of those with higher bandwidths.

ƒ 1. IDEAL OP AMP ATTRIBUTES

ƒ- Infinite Differential Gain

- Zero Common Mode Gain

- ƒ Zero Offset Voltage ƒ

- Zero Bias Current ƒ

- Infinite Bandwidth

2.ƒ OP AMP INPUT ATTRIBUTES

ƒ- Infinite Impedance ƒ

- Zero Bias Current ƒ

- Respond to Differential Voltages ƒ

- Do Not Respond to Common Mode Voltages

3.ƒ OP AMP OUTPUT ATTRIBITES ƒ

- Zero Impedance

The basic operation of the op amp can be easily summarized. First we assume that there is a portion of the output that is fed back to the inverting terminal to establish the fixed gain for the amplifier. This is negative feedback. Any differential voltage across the input terminals of the op amp is multiplied by the amplifier’s open-loop gain. If the magnitude of this differential voltage is more positive on the inverting (-) terminal than on the non-inverting (+) terminal, the output will go more negative. If the magnitude of the differential voltage is more positive on the non-inverting (+) terminal than on the inverting (-) terminal, the output voltage will become more positive. The open-loop gain of the amplifier will attempt to force the differential voltage to zero. As long as the input and output stays in the operational range of the amplifier, it will keep the differential voltage at zero, and the output will be the input voltage multiplied by the gain set by the feedback. Note from this that the inputs respond to differential mode not common-mode input voltage.

There are two basic ways to configure the voltage feedback op amp as an amplifier.

The figure above shows what is known as the inverting configuration. With this circuit, the output is out of phase with the input. The gain of this circuit is determined by the ratio of the resistors used and is given by:

A= -R2/R1

The figure above shows the simulation of the inverting OPAMP in circuits-cloud simulator

http://circuits-cloud.com/

with V1=10 volt

A= -R2/R1 = -10/10 = -1

V(out)= A * V(in) = -1 * 10 = -10 volt

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This is the link of the circuit:

http://circuits-cloud.com/circuit/editor/d7a79de84...

The figure above shows what is know as the non-inverting configuration. With this circuit the output is in phase with the input. The gain of the circuit is also determined by the ratio of the resistors used and is given by:

A= 1+ (R2/R1)

The figure above shows the simulation of non-inverting OPAMP in circuits-cloud simulator

http://circuits-cloud.com/

with V1=10 volt

A= 1+ (R2/R1) = 1+ (10/10) = 2

V(out)= A * V(in) = 2 * 10 = 20 volt

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and this is the link of circuit

http://circuits-cloud.com/circuit/editor/a2414dc2e...

Not all amplifiers are the same and there is a clear distinction made between the way their output stages operate. The main operating characteristics of an ideal amplifier are linearity, signal gain, efficiency and power output but in real world amplifiers there is always a trade off between these different characteristics.

Amplifier Classes is the term used to differentiate between the different amplifier types.

Class A Amplifiers are the most common type of amplifier class due mainly to their simple design. Class A, literally means “the best class” of amplifier due mainly to their low signal distortion levels and are probably the best sounding of all the amplifier classes mentioned here. The class A amplifier has the highest linearity over the other amplifier classes and as such operates in the linear portion of the characteristics curve.

Generally class A amplifiers use the same single transistor (Bipolar, FET, IGBT, etc) connected in a common emitter configuration for both halves of the waveform with the transistor always having current flowing through it, even if it has no base signal. This means that the output stage whether using a Bipolar, MOSFET or IGBT device, is never driven fully into its cut-off or saturation regions but instead has a base biasing Q-point in the middle of its load line. Then the transistor never turns “OFF” which is one of its main disadvantages.

To achieve high linearity and gain, the output stage of a class A amplifier is biased “ON” (conducting) all the time. Then for an amplifier to be classified as “Class A” the zero signal idle current in the output stage must be equal to or greater than the maximum load current (usually a loudspeaker) required to produce the largest output signal.
As a class A amplifier operates in the linear portion of its characteristic curves, the single output device conducts through a full 360 degrees of the output waveform. Then the class A amplifier is equivalent to a current source. Since a class A amplifier operates in the linear region, the transistors base (or gate) DC biasing voltage should by chosen properly to ensure correct operation and low distortion. However, as the output device is “ON” at all times, it is constantly carrying current, which represents a continuous loss of power in the amplifier. Due to this continuous loss of power class A amplifiers create tremendous amounts of heat adding to their very low efficiency at around 30%, making them impractical for high-power amplifications. Also due to the high idling current of the amplifier, the power supply must be sized accordingly and be well filtered to avoid any amplifier hum and noise. Therefore, due to the low efficiency and over heating problems of Class A amplifiers, more efficient amplifier classes have been developed

Class B amplifiers were invented as a solution to the efficiency and heating problems associated with the previous class A amplifier. The basic class B amplifier uses two complimentary transistors either bipolar of FET for each half of the waveform with its output stage configured in a “push-pull” type arrangement, so that each transistor device amplifies only half of the output waveform.

In the class B amplifier, there is no DC base bias current as its quiescent current is zero, so that the dc power is small and therefore its efficiency is much higher than that of the class A amplifier. However, the price paid for the improvement in the efficiency is in the linearity of the switching device.

When the input signal goes positive, the positive biased transistor conducts while the negative transistor is switched “OFF”. Likewise, when the input signal goes negative, the positive transistor switches “OFF” while the negative biased transistor turns “ON” and conducts the negative portion of the signal. Thus the transistor conducts only half of the time, either on positive or negative half cycle of the input signal.

Then we can see that each transistor device of the class B amplifier only conducts through one half or 180 degrees of the output waveform in strict time alternation, but as the output stage has devices for both halves of the signal waveform the two halves are combined together to produce the full linear output waveform. This push-pull design of amplifier is obviously more efficient than Class A, at about 50%, but the problem with the class B amplifier design is that it can create distortion at the zero-crossing point of the waveform due to the transistors dead band of input base voltages from -0.7V to +0.7.

This zero-crossing distortion (also known as Crossover Distortion)

class B amplifier in circuits cloud:
http://circuits-cloud.com/circuit/details/1GcqzYKh...

we will design the class B amplifier to see the crossover distortion

1. visit the circuits-cloud simulator
http://circuits-cloud.com/

2. Select circuit editor. Window of circuits cloud Workspace will open.

3. Choose the analog type of simulation.

4. The connection shown above, just drag and drop the components you need and then connect between them.

5. Make sure all components has been connected.

- In order to simulate your circuit you should edit the parameters

- the editing done by double click on the parameter > edit > save parameter

example:

1. AC source

voltage = 5 volt

frequency = 1000 Hz

2. NPN transistor

model = 2N222

3. The two DC sources

voltage = 10 volt

4. Resistor

Resistance= 10K 5.

double click on the wires that you should get the input and output on it.

put a name and click ( enable prob) then save.

- save the circuit

- click on Run > create new simulation

- decide when your signal should start , when it will stop and the time per one step.

for example:

start time = 0

stop time = 5ms

Time step = 10us

- click on ( create and Run)

- Now the simulation will appears and you will see the input and output signals

To overcome The Crossover Distortion, class AB amplifiers were developed.

The Class AB Amplifier is a combination of the “Class A” and the “Class B” type amplifiers . The AB classification of amplifier is currently one of the most common used types of audio power amplifier design. The class AB amplifier is a variation of a class B amplifier , except that both devices are allowed to conduct at the same time around the waveforms crossover point eliminating the crossover distortion problems of the previous class B amplifier.
The two transistors have a very small bias voltage, typically at 5 to 10% of the quiescent current to bias the transistors just above its cut-off point. Then the conducting device, either bipolar of FET, will be “ON” for more than one half cycle, but much less than one full cycle of the input signal. Therefore, in a class AB amplifier design each of the push-pull transistors is conducting for slightly more than the half cycle of conduction in class B, but much less than the full cycle of conduction of class A. In other words, the conduction angle of a class AB amplifier is somewhere between 180 and 360 depending upon the chosen bias point as shown.

The advantage of this small bias voltage, provided by series diodes or resistors, is that the crossover distortion created by the class B amplifier characteristics is overcome, without the inefficiencies of the class A amplifier design. So the class AB amplifier is a good compromise between class A and class B in terms of efficiency and linearity, with conversion efficiencies reaching about 50% to 60%.

The Class C Amplifier design has the greatest efficiency but the poorest linearity of the classes of amplifiers mentioned here. The previous classes, A, B and AB are considered linear amplifiers, as the output signals amplitude and phase are linearly related to the input signals amplitude and phase.

However, the class C amplifier is heavily biased so that the output current is zero for more than one half of an input sinusoidal signal cycle with the transistor idling at its cut-off point. In other words, the conduction angle for the transistor is significantly less than 180 degrees, and is generally around the 90 degrees area. While this form of transistor biasing gives a much improved efficiency of around 80% to the amplifier, it introduces a very heavy distortion of the output signal. Therefore, class C amplifiers are not suitable for use as audio amplifiers.

Due to its heavy audio distortion, class C amplifiers are commonly used in high frequency sine wave oscillators and certain types of radio frequency amplifiers, where the pulses of current produced at the amplifiers output can be converted to complete sine waves of a particular frequency by the use of LC resonant circuits in its collector circuit.