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Op amp non investing voltage amplifier transistor

op amp non investing voltage amplifier transistor

In the most basic circuit, op-amps are used as voltage amplifiers, which can be divided into noninverting and inverting amplifiers. The inverting input is denoted with a minus (-) sign, and the non-inverting input uses a positive (+) sign. Operational amplifiers work to amplify the voltage. In this circuit, the transistor is active all the time. The op-amp operates as a non-inverting amplifier with the transistor as current booster. FANTASY HORSE RACING BETTING RULES

The input signal is applied at the non-inverting input of the opamp. A non-inverting amplifier also acts as a voltage follower circuit. The non-inverting amplifiers also have negative feedback which is used to control the gain of the amplifier. Feedback contains a voltage divider circuit that provides a part of the output to the input terminal.

This makes it an ideal buffer. The input and output voltages are in phase with each other, their phase difference is 0 or degrees. The feedback resistor Rf introduces negative feedback at the negative inverting input terminal of the opamp. Many have input stages which behave in unexpected ways when driven with large differential voltages, in fact, in many cases, the differential input voltage range of the op amp is limited.

And op amp outputs are rarely compatible with logic. Yet many designers still try to use op amps as comparators. While this may work at low speeds and low resolutions, many times the results are not satisfactory. Not all of the issues involved with using an op amp as a comparator can be resolved by reference to the op amp data sheet, since op amps are not intended for use as comparators. The most common issues are speed as we have already mentioned , the effects of input structures protection diodes, phase inversion in FET amplifiers, and many others , output structures which are not intended to drive logic, hysteresis and stability, and common-mode effects.

Why should we expect low speed when using an op amp as a comparator? A comparator is designed to be used with large differential input voltages, whereas op amps normally operate with their differential input voltage minimized by negative feedback.

When an op amp is over-driven, sometimes by as little as a few millivolts, some of the internal stages may saturate. If this occurs the device will take a comparatively long time to come out of saturation and will therefore be much slower than if it always remained unsaturated see figure 4. The time to come out of saturation of an overdriven op amp is likely to be considerably longer than the normal group delay of the amplifier, and will often depend on the amount of overdrive.

Since few op amps have this saturation recovery time specified for various amounts of overdrive it will generally be necessary to determine, by experimental measurements in the lab, the behavior of the amplifier under the conditions of overdrive to be expected in a particular design. The results of such experimental measurements should be regarded with suspicion and the values of propagation delay through the op amp comparator which is chosen for worst-case design calculations should be at least twice the worst value seen in any experiment.

Frequently the logic being driven by the op amp comparator will not share the op amp's supplies and the op amp rail to rail swing may go outside the logic supply rails-this will probably damage the logic circuitry, and the resulting short circuit may damage the op amp as well. ECL is a very fast current steering logic family. It is unlikely that an op amp would be used as a comparator in applications where ECL's highest speed is involved, for reasons given above, so we shall usually be concerned only to drive ECL logic levels from an op amp's signal swing and some additional loss of speed due to stray capacities will be unimportant.

To do this we need only three resistors, as shown in figure 4. R1, R2 and R3 are chosen so that when the op amp output is positive the level at the gate is Using low resistance values for R1, R2 and R3 will minimize the effects of stray capacitance but at the same time will increase power consumption. A resistor between the op amp output and the MOS FET gate and the diode to ground are generally not needed left side of figure 4.

The speed of the transition depends on the value of RL and the stray capacity of the output node. The lower the value of RL the faster the response will be, but the higher the power consumption. Furthermore, it may be made inverting or non-inverting by simple positioning of components. It does, however, have a large current surge during switching, when both devices are on at once, and unless MOS devices with high channel resistance are used a current limiting resistor may be necessary to reduce this effect.

It is also important, in this application and the one in figure 4. The first-level assumption engineers make about all op amps and comparators is that they have infinite input impedance and can be regarded as open circuits except for current feedback transimpedance op amps, which have a high impedance on their non-inverting input but a low impedance of a few tens of ohms on their inverting input But many op amps especially bias-compensated ones such as the OP and its many descendants contain protective circuitry to prevent large differential input voltages from damaging the input stage transistors.

Protective circuitry such as current limiting resistors and clamp diodes, as shown in figure 4. Other op amp designs contain more complex input circuitry, which only has high impedance when the differential voltage applied to it is less than a few tens of mV , or which may actually be damaged by differential voltages of more than a few volts.

It is therefore necessary, when using an op amp as a comparator, to study the manufacturer's data sheet to determine how the input circuitry behaves when large differential voltages are applied to it. It is always necessary to study the data sheet when using an integrated circuit to ensure that its non-ideal behavior, and every integrated circuit ever made has some non-ideal behavior, is compatible with the proposed design - it is just more important than usual in the present case.

Of course some comparator applications never involve large differential voltages-or if they do the comparator input impedance when large differential voltages are present is comparatively unimportant. In such cases it may be appropriate to use as a comparator an op amp whose input circuitry behaves non-linearly-but the issues involved must be considered, not just ignored.

Their inverting and non-inverting inputs may become interchanged. If this should occur when the op amp is being used as a comparator the phase of the system involved will be inverted, which could well be inconvenient. The solution is, again, careful reading of the data sheet to determine just what common-mode range is acceptable. Also, the absence of negative feedback means that, unlike that of op amp circuits, the input impedance is not multiplied by the loop gain.

As a result, the input current varies as the comparator switches. Therefore the driving impedance, along with parasitic feedback paths, can play a key role in affecting circuit stability. While negative feedback tends to keep amplifiers within their linear region, positive feedback forces them into saturation.

Section Summary Operational amplifiers are not designed to be used as comparators, so this section has been, intentionally, a little discouraging. Nevertheless there are some cases where the use of an op amp as a comparator is a useful engineering decision-what is important is to make it a considered decision, and ensure that the op amp chosen will perform as expected.

To do this it is necessary to read the manufacturer's data sheet carefully, to consider the effects of non-ideal op amp performance, and to calculate the effects of op amp parameters on the overall circuit. Since the op amp is being used in a non-standard manner some experimentation may also be necessary, since the amplifier used for the experiment will not necessarily be typical and the results of experiments should always be interpreted somewhat pessimistically.

This can result in the possibility that the output will switch back and forth several times as the input transitions through the comparator threshold voltage. The very large open loop gain of the amplifier will allow only small levels of noise on the input to cause the output to change.

This may not cause a problem in some circumstances, but if the output from the operational amplifier comparator is being fed into fast logic circuitry, then it can often result in problems. For example, if the desire is to count the number of times the input crosses the threshold then these multiple output changes per input transition will give false readings. The problem can be solved very easily by adding some positive feedback to the operational amplifier or comparator circuit.

This is provided by the addition of R3 in the circuit in figure 4. The circuit is known as a Schmitt trigger. Resistor divider R1 and R2 set the comparison voltage at the non-inverting input of the op amp. When the output of the comparator is high, this voltage is fed back to the non-inverting input of the op amp or comparator.

As a result the comparison threshold becomes higher. When the output is switched low, the comparison threshold is lowered. This gives the circuit what is called hysteresis. It is straight forward to calculate the resistor values needed for the Schmitt trigger circuit. The center voltage about which the circuit will switch is determined by the voltage divider consisting of R1 and R2.

This should be chosen first. Then the feedback resistor R3 can be calculated. This will provide a level of hysteresis that is equal to the output swing of the op amp reduced by the voltage divider attenuation formed as a result of R3 and the parallel combination of R1 and R2. The higher the value of R3 with respect to R1 R2 the smaller the hysteresis, or the difference between the two threshold levels.

The fact that the positive feedback applied within the circuit ensures that there is effectively a higher gain and therefore the switching is faster. This is particularly useful when the input waveform may be slow.

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01 - The Non-Inverting Op-Amp (Amplifier) Circuit


When the input is at zero volts, Q1 is cut off, so Q2 is driven fully on via R2, and the output is low saturated. When the input is at zero volts, Q1 is cut off, so Q2 is also cut off via R2-R3 and the output is at zero volts. Under this condition, the output takes up a value a few hundred mV below the positive supply rail value. The circuit operates as follows.

Figures 6 through 8 show examples of such circuits. R1 gives base drive protection, and can be larger than 1k0, if desired. The relay is turned on by a positive input voltage. Simple relay-driving circuit The current sensitivity of the relay can be raised by a factor of about 20, by replacing Q1 with a Darlington-connected pair of transistors. Figure 7 shows this technique used to make a circuit that can be activated by placing a resistance of less than 2M0 across a pair of stainless metal probes.

Water, steam, and skin contacts have resistances below this value, so this simple little circuit can be used as a water, steam, or touch-activated relay switch. R2 ensures that Q1 and Q2 turn completely off when the input terminals are open circuit. Simple npn common-emitter amplifier The first step in designing a circuit of the basic Figure 9 type is to select the value of load resistor R2.

In the example shown, the input impedance is roughly 5k0, and is shunted by R1 — the voltage gain works out at about x, or 46dB. The feedback action is such that any shift in the output level due to variations in hfe, temperature, or component values causes a counter-change in the base-current biasing level, thus tending to cancel the original shift. Common-emitter amplifier with feedback biasing The Figure 10 circuit has the same values of bandwidth and voltage gain as the Figure 9 design, but has a lower total value of input impedance.

If desired, the shunting effects of the biasing network can be eliminated by using two feedback resistors and AC-decoupling them as shown in Figure Alternative gain values can be obtained by altering the R5 value. Fixed-gain x10 common-emitter amplifier Figure 14 shows a useful variation of the above design. In this case, R3 equals R4, and is not decoupled, so the circuit gives unity voltage gain. Note, however, that this circuit gives two unity-gain output signals, with the emitter output in phase with the input and the collector signal in anti-phase.

This circuit thus acts as a unity-gain phase splitter. Unity-gain phase splitter Figure 15 shows another way of varying circuit gain. Alternative fixed-gain x10 amplifier Finally, Figure 16 shows how the Figure 10 design can be modified to give a wide-band performance by wiring DC-coupled emitter follower buffer Q2 between Q1 collector and the output terminal, to minimize the shunting effects of stray capacitance on R2, and thus extending the upper bandwidth to several hundred kHz.

When the non-inverting input is connected to the ground, i. Since the inverting input terminal is at ground level, the junction of the resistors R1 and R2 must also be at ground level. This implies that the voltage drop across R1 will be zero. As a result, the current flowing through R1 and R2 must be zero. Thus, there are zero voltage drops across R2, and therefore the output voltage is equal to the input voltage, which is 0V. When a positive-going input signal is applied to the non-inverting input terminal, the output voltage will shift to keep the inverting input terminal equal to that of the input voltage applied.

The closed-loop voltage gain of a non-inverting amplifier is determined by the ratio of the resistors R1 and R2 used in the circuit. Practically, non-inverting amplifiers will have a resistor in series with the input voltage source, to keep the input current the same at both input terminals. Virtual Short In a non-inverting amplifier, there exists a virtual short between the two input terminals.

A virtual short is a short circuit for voltage, but an open-circuit for current. The virtual short uses two properties of an ideal op-amp: Since RIN is infinite, the input current at both the terminals is zero. Although virtual short is an ideal approximation, it gives accurate values when used with heavy negative feedback. As long as the op-amp is operating in the linear region not saturated, positively or negatively , the open-loop voltage gain approaches infinity and a virtual short exists between two input terminals.

Because of the virtual short, the inverting input voltage follows the non-inverting input voltage. If the non-inverting input voltage increases or decreases, the inverting input voltage immediately increases or decreases to the same value. In other words, the gain of a voltage follower circuit is unity.

The output of the op-amp is directly connected to the inverting input terminal, and the input voltage is applied at the non-inverting input terminal. The voltage follower, like a non-inverting amplifier, has very high input impedance and very low output impedance. The circuit diagram of a voltage follower is shown in the figure below.

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Op-amps 3: Non-inverting Amp Voltage Gain Derivation

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