### Two stage amplifier circuit analysis

The op amp circuit is a powerful took in modern circuit applications. You can put together basic op amp circuits to build mathematical models that predict complex, real-world behavior.

Commercial op amps first entered the market as integrated circuits in the mids, and by the early s, they dominated the active device market in analog circuits.

The op amp itself consists of a complex arrangement of transistors, diodes, resistors, and capacitors put together and built on a tiny silicon chip called an integrated circuit. You just need some basic knowledge of the constraints on the voltages and currents at the external terminals of the device.

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Unlike capacitors, inductors, and resistors, op amps require power to work. Op amps have the following five key terminals, shown here:. The positive terminal, called the noninverting input v P. You can model the op amp with a dependent source if you need accurate results, but the ideal op amp is good enough for most applications. The op amp amplifies the difference between the two inputs, v P and v Nby a gain A to give you a voltage output v O :.

When the output voltage exceeds the supplied power, the op amp saturates.

### Design and Analysis of A Single Stage Transistor Amplifier Using C++

This means that the output is clipped or maxed out at the supplied voltages and can increase no further. When this happens, the op amp behavior is no longer linear but operates in the nonlinear region. You can see this idea here; the left diagram shows the transfer characteristic, whereas the right diagram shows the ideal transfer characteristic of an op amp with an infinite gain.

The graph shows three modes of operation for the op amp. You have positive and negative saturated regions, showing the nonlinear and linear regions. If you want to make signals bigger, you need to operate in the linear region.

You can describe the three regions mathematically as follows. To perform math functions such as addition and subtractionthe op amp must work in linear mode.

All op amp circuits shown here operate in the linear active region. If you need accurate results, you can model the op amp with a voltage-controlled dependent source, like the one shown here.

This model consists of a large gain A, a large input resistance R Iand a small output resistance R O. The table shows ideal and typical values of these op amp properties. As long as the op amp has high gain, the op amp math circuits will work. High-input resistance draws little current from the input source circuit, increasing battery life for portable applications.

Low- or no-output resistance delivers maximum voltage to the output load. The dependent voltage-controlled current source is shown here as well. The output is restricted between the positive and negative voltages when the op amp is operating in the linear region. These equations make analyzing op amps a snap and provide you with valuable insight into circuit behavior. Because feedback from the output terminals to one or both inputs ensures that v P and v N are equal.

To get the first constraint, consider that the linear region of an op amp is governed by when the output is restricted by the supply voltages as follows:. You can rearrange the equation to limit the input to v P — v N :. An op amp with infinite gain will always have the noninverting and inverting voltages equal.

This equation becomes useful when you analyze a number of op amp circuits, such as the op amp noninverter, inverter, summer, and subtractor.

## Two-stage BJT & FET amplifier

The other important op amp equation takes a look at the input resistance R I. An ideal op amp has infinite resistance. This implies that no input currents can enter the op amp:.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service. Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.

I have designed a two-stage amplifier using 2N's. To be clear and to protect from future edits to the question, here is the circuit you are asking about:. You want a gain ofso at best that means a gain of 71 from each stage.

That is too much gain to expect a single-transistor stage to have. You therefore should be expecting at least 3 stages. If the gain is balanced between stages, that would mean a gain of 17 per stage. That is more manageable, but still has problems. Even with a gain of 17 per stage, the gain won't be all that well controlled.

A better way is to make more open loop gain, then use negative feedback to set the closed loop gain to what you really want. The feedback ratio, which defines that closed loop gain, can be set by two resistors.

This allows good predictability and flat frequency response. Transistors are cheap and small, so there is no reason to economize the number of transistors. If you just want something like a microphone amplifier and the result is more important than the journey, just use a couple of opamps. Just number them next time. The approximation of constant B-E voltage is no longer valid.

You have to keep the capabilities of the transistors in mind. If the intent is to have a reasonably linear amplifier, don't try to get more than a gain of 20 out of a single stage like that. A gain of 10 would be better, especially if there is no global negative feedback. C1 will cause a high pass filter working against Rs. Ignoring the additional input impedance of the first stage, that comes out to Hz. Is that what you intended?

If you care about lower frequencies, then you have to re-think C1. To be fair, the input impedance of the first stage adds to the effective resistance that C1 is working against, but that is rather unpredictable. It's not going to be very high due to the very low RE1A.A real device deviates from a perfect difference amplifier. One minus one may not be zero. It may have have an offset like an analog meter which is not zeroed.

The inputs may draw current. The characteristics may drift with age and temperature. Gain may be reduced at high frequencies, and phase may shift from input to output. These imperfection may cause no noticable errors in some applications, unacceptable errors in others. In some cases these errors may be compensated for. Sometimes a higher quality, higher cost device is required.

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As stated before, an ideal differential amplifier only amplifies the voltage difference between its two inputs. If the two inputs of a differential amplifier were to be shorted together thus ensuring zero potential difference between themthere should be no change in output voltage for any amount of voltage applied between those two shorted inputs and ground:.

This translates to a common-mode voltage gain of zero. The operational amplifier, being a differential amplifier with high differential gain, would ideally have zero common-mode gain as well.

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In real life, however, this is not easily attained. The performance of a real op-amp in this regard is most commonly measured in terms of its differential voltage gain how much it amplifies the difference between two input voltages versus its common-mode voltage gain how much it amplifies a common-mode voltage. The ratio of the former to the latter is called the common-mode rejection ratioabbreviated as CMRR:.

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An ideal op-ampwith zero common-mode gain would have an infinite CMRR. Real op-amps have high CMRRs, the ubiquitous having something around 70 dB, which works out to a little over 3, in terms of a ratio.

Because the common mode rejection ratio in a typical op-amp is so high, common-mode gain is usually not a great concern in circuits where the op-amp is being used with negative feedback. If the common-mode input voltage of an amplifier circuit were to suddenly change, thus producing a corresponding change in the output due to common-mode gain, that change in output would be quickly corrected as negative feedback and differential gain being much greater than common-mode gain worked to bring the system back to equilibrium.

Sure enough, a change might be seen at the output, but it would be a lot smaller than what you might expect. A consideration to keep in mind, though, is common-mode gain in differential op-amp circuits such as instrumentation amplifiers. We should expect to see no change in output voltage as the common-mode voltage changes:. Aside from very small deviations actually due to quirks of SPICE rather than real behavior of the circuitthe output remains stable where it should be: at 0 volts, with zero input voltage differential.

Our input voltage differential is still zero volts, yet the output voltage changes significantly as the common-mode voltage is changed. More than that, its a common-mode gain of our own making, having nothing to do with imperfections in the op-amps themselves. With a much-tempered differential gain actually equal to 3 in this particular circuit and no negative feedback outside the circuit, this common-mode gain will go unchecked in an instrument signal application.The design includes PNP and NPN transistors and adopts the overall topology of the Sziklai pair, but with additional resistors included to define the gain.

The two transistor amplifier offers a reasonably high impedance while providing a low output impedance. It is an ideal transistor amplifier circuit for applications where a higher level of gain is required than that which would be provided by a single transistor stage. The resistors R1 and R2 are chosen to set the base of TR1 to around the mid point.

If some current limiting is required then it is possible to place a resistor between the emitter of TR2 and the supply. The two transistor amplifier is a useful design to have in the electronics engineers toolbox. It is a simple circuit, yet operates effectively in scenarios where a little mroe gain is required than that which could be provided by a single transistor. Supplier Directory For everything from distribution to test equipment, components and more, our directory covers it. Selected Video What is an Op Amp? Featured articles.A single stage of amplifier can provide only a limited current gain or voltage gain. Most of the applications require much higher gain. Hence, we usually use several amplifier stages connected in cascade i.

Thus, a multistage amplifier or cascade amplifier may provide a higher voltage gain or current gain. Cascading of amplifier stages is usually done to increase the total gain of the amplifier. However, sometimes cascading is done to get the desired output and input impedance for specific applications.

Figure 1 gives the block diagram of two-stage amplifier. The first stage is driven by a voltage source V s having series source resistance R s.

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Alternatively, the driving source may be current source I s with shunt resistance R s. Actual voltage available at the input of the first stage is V i while V 0 is the output voltage of the second stage.

Then the ratio forms the voltage gain of the two stage cascade amplifier. Instead of two stages as shown in figure 3, we may have three or more stages connected in cascade amplifier, it becomes possible to amplify a weak input voltage V i of just a few microvolts to get an output voltage V 0 of several volts. Transistor amplifier may be connected in any of the three configurations namely common emitter CEcommon base CB and common collector CC.

However, in cascade amplifier meant for providing high gain, only CE amplifier stage are connected in cascade. CB and CC configurations can not be used for this purpose. Figure 2 gives the circuit of a two stage CE audio amplifier.

The circuit gives the typical biasing arrangement and use of coupling capacitors C b1 and C b2.

### Simple two transistor amplifier

Typical values of circuit components are also given. The load impedance is a resistor while coupling is through a capacitor. Hence this cascade amplifier forms the so-called resistance — Capacitance coupled or RC coupled amplifier.For most systems a single transistor amplifier does not provide sufficient gain or bandwidth or will not have the correct input or output impedance matching. The solution is to combine multiple stages of amplification. We have the three basic one transistor amplifier configurations to use as building blocks to create more complex amplifier systems which can provide better optimized specifications and performance.

The sections in this chapter tend to use BJT devices to illustrate the circuit concepts but these multi-stage amplifiers can be constructed from MOS FET devices, or a combination, just as easily and the methods used to analyze them are much the same as well.

Design of two stage operational amplifier (opamp) part 1

It is necessary to consider what happens when non-ideal amplifiers are put in series. Looking at the example in figure For the above example, let us now calculate the gain assuming nothing about the R in and R out of each stage, treating them as voltage dividers between the two stages and between the last stage and the output load.

Note that in practice, impedances, Z inZ outwould normally be used, not resistances, but the simple resistance will serve to illustrate the point here. As a matter of fact, we really only need R out to go to 0 to have the resistor dividers to go to 1. The above equations assume that the individual amplifier gains, A do not change with output loading. That effect, if any, is modeled in the R out.

By the time you reached that point, other adverse effects would have caused much more trouble, for example, the fact that noise from each successive stage is added to the noise coming into that stage and is further amplified on down the cascade of amplifiers.

Even if AC-coupled, noise from preceding stages gets amplified by each downstream amplifier stage, making for nothing but a noise source after a while. We generally refer all noise to the input of the signal chain, taking out the effects of the gain stages. The impact of input and output loading can be minimized by cascading two amplifiers with appropriate input and output characteristics.

Multistage cascading can be used to create amplifiers with high input resistance, low output resistance and large gains.

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The cascade of a Common Emitter amplifier stage followed by a Common Collector emitter-follower amplifier stage can provide a good overall voltage amplifier, figure The Common Emitter input resistance is relatively high and Common Collector output resistance is relatively low. The voltage follower second stage, Q 2contributes no increase in voltage gain but provides a near voltage-source low resistance output so that the gain is nearly independent of load resistance.

The high input resistance of the Common Emitter stage, Q 1makes the input voltage nearly independent of input-source resistance. Multiple Common Emitter stages can be cascaded with emitter follower stages inserted between them to reduce the attenuation due to inter-stage loading.Other direct-coupled circuits can be converted into negative feedback amplifiers by following the same procedure.

As always, the best approach is to first design the circuit as a non-feedback amplifier, then determine the feedback component values. Note that there is no coupling-capacitor for the feedback network in the circuit shown in This is the most economical of all two-stage BJT amplifier circuits, because it has the smallest quantity of components.

It can have just as high a voltage gain open-loop and closed-loop as any other two-stage circuit. As a negative feedback amplifier, its input resistance is normally higher than that for a BJT circuit using voltage divider bias, R 2 is usually larger than the parallel resistance of voltage divider resistors.

As always, the feedback components are determined after the circuit is designed for the largest possible open-loop voltage gain. The lower cutoff frequency for the circuit in Fig. The modification shown in Fig. In this case, emitter resistor R 5 affects the circuit dc bias conditions, so it must be designed into the circuit from the start. As in other circuits, it might be possible to omit the feedback network coupling capacitor, C F1 in Fig. Therefore, BIFET negative feedback amplifiers must be designed for relative small closed-loop voltage gains, usually a maximum of around The single major advantage of BIFET circuits, very high input impedance, is largely unchanged by negative feedback.

Therefore, to determine C S. Search Topics Here … Search for:. February 26, Amplifiers with Negative Feedback.