Diff amp with current mirror amplifier

For designing the monolithic integrated circuit , the most popular technique is used namely the current mirror. So in this method, the designing of the circuit can be done to copy the flow of current throughout one energetic device to another including the feature of current control. Here, the flow of current can be copied in the form of inverting from device to device. Once the flow of current within the first active device is altered then the reflected output current from the other active device will also be changed. This article discusses an overview of the current mirror circuit and its working. The circuit is used to copy the flow of current in one active device and controlling the flow of current in another device by maintaining the output current stable instead of loading is known as a current mirror.

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Diff amp with current mirror amplifier

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• What is a Current Mirror : Circuit & Its Working
• Differential amplifier
• Design of Differential Amplifier Using Current Mirror Load in 90 nm CMOS Technology
• US4194166A - Differential amplifier with a current mirror circuit - Google Patents
• US7545216B2 - Amplifier with current mirror bias adjust - Google Patents
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WATCH RELATED VIDEO: DIFFERENTIAL AMPLIFIER AND CURRENT MIRROR

What is a Current Mirror : Circuit & Its Working

Most modern operational amplifiers utilize a differential amplifier front end. In other words, the first stage of the operational amplifier is a differential amplifier. This circuit is commonly referred to as a diff amp or as a long-tailed pair. A diff amp utilizes a minimum of 2 active devices, although 4 or more may be used in more complex designs.

Our purpose here is to examine the basics of the diff amp so that we can understand how it relates to the larger operational amplifier. Therefore, we will not be investigating the more esoteric designs. To approach this in an orderly fashion, we will examine the DC analysis first, and then follow with the AC small signal analysis. Note the inherent symmetry of the circuit. If you were to slice the circuit in half vertically, all of the components on the left half would have a corresponding component on the right half.

Indeed, for optimal performance, we will see that these component pairs should have identical values. For critical applications, a matched pair of transistors would be used. This is in essence, an emitter bias technique. Given identical emitter currents, it follows that the remaining currents and voltages in the two halves must be identical as well. You may assume that the two transistors are very closely matched. If we make the approximation that collector and emitter currents are equal, we may find the collector voltage by calculating the voltage drop across the collector resistor, and subtracting the result from the positive power supply.

This result indicates that the actual emitter voltage is closer to This error is probably within the error we can expect by using the 0. As you have no doubt guessed, it is impossible to make both halves of the circuit identical, and thus, the currents and voltages will never be exactly the same. Even a small resistor tolerance variation will cause an upset.

If the base resistors are mismatched, this will cause a direct change in the two base potentials. A variation in collector resistance will cause a mismatch in the collector potentials. In simple terms, the difference between the two base currents is the input offset current.

The difference between the two collector voltages is the output offset voltage. The DC potential required at one of the bases to counteract the output offset voltage is called the input offset voltage this is little more than the output offset voltage divided by the DC gain of the amplifier. In an ideal diff amp all three of these factors are equal to 0. We will take a much closer look at these parameters and how they relate to operational amplifiers in later chapters. For now, it is only important that you understand that these inaccuracies exist, and what can cause them.

In order to minimize confusion with the DC circuit, AC equivalent values will be shown in lower case. This circuit has two signal inputs and two signal outputs. This means that there are four variations on the theme:. Because the diff amp is a linear circuit, we can use the principle of Superposition to independently determine the output contribution from each of the inputs. For the output on collector 1, transistor 1 forms the basis of a common emitter amplifier.

The negative sign comes from the fact that AC ground is used as our reference. To a reasonable approximation, we can say that the collector and emitter currents are identical. You may recall the following Equation from your prior course work:. By definition, the AC emitter current must equal the AC emitter potential divided by the AC resistance in the emitter section. Because the circuit values should be symmetrical for best performance, this Equation may be simplified to.

The final negative sign indicates that the collector voltage at transistor number 1 is degrees out of phase with the input signal. Because of this, the magnitude of the collector voltage at transistor number 2 will be the same as that on the first transistor. Because the second current is out of phase with the first, it follows that the second collector voltage must be out of phase with the first.

This means that the voltage at the second collector is in phase with the first input signal. Its gain Equation is. The output signal will be in phase if we are examining the opposite transistor, and out of phase if we are looking at the input transistor.

Because the circuit is symmetrical, we will get similar results when we examine the second input. The voltage between the two collectors is degrees apart. If we were to use a differential output, that is, derive the output from collector to collector rather than from one collector to ground, we would see an effective doubling of the output signal. If the reason for this is not clear to you, consider the following.

Assume that each collector has a 1 V peak sine wave riding on it. Likewise, when collector 1 is at its negative peak, collector 2 is at its positive peak, producing a total of -2 V. Because it is possible to drive a diff amp with two distinct inputs, a wide variety of outputs may be obtained. It is useful to investigate two specific cases:. Using Superposition, we find that the outputs due to each input are times 10 mV, or 1 V in magnitude. Note that each collector sees both a sine wave and an inverted sine wave, both of equal amplitude.

In Equation form,. The exact same effect is seen on the opposite collector. This last Equation is very important. It says that the output voltage is equal to the gain times the difference between the two inputs. This is how the differential amplifier got its name. In this case, the two inputs are identical, and thus their difference is zero. On the other hand, if we were to invert one of the input signals case 2 , we find a completely different result.

Thus, if one input is inverted, the net result is a doubling of gain. In short, a differential amplifier suppresses in phase signals while simultaneously boosting out of phase signals. This can be a very useful attribute, particularly in the area of noise reduction.

By convention, in phase signals are known as common-mode signals. An ideal differential amplifier will perfectly suppress these common-mode signals, and thus, its common-mode gain is said to be zero. In the real world, a diff amp will never exhibit perfect common-mode rejection. The common-mode gain may be made very small, but it is never zero. For a common-mode gain of zero, the two halves of the circuit have to be perfectly matched, and all circuit elements must be ideal.

This is impossible to achieve as errors may arise from several sources. The most obvious error sources are resistor tolerance variations and transistor parameter spreads. The basic design of the circuit will also affect the common-mode gain. With some circuit rearrangements, it is possible to determine a common-mode gain for the circuits we have been using. This circuit has been effectively reduced to a simple common emitter stage. Based on our earlier work, the gain for this circuit is.

This is the common-mode voltage gain. A very high internal resistance i. There are many ways of creating a more ideal current source. This can help to reduce temperature induced current fluctuations. In any case, the effective resistance of this current source is considerably larger than the simple tail resistor variation.

It is largely dependent on the characteristics of the tail current transistor, and can easily be in the megohm region. A very popular biasing technique in integrated circuits involves the current mirror. This circuit requires that the transconductance curves of the diode and the transistor be very closely matched. One way to guarantee this is to use two transistors, and form one of them into a diode by shorting its collector to its base. If we use an approximate forward bias potential of 0.

In reality, the diode potential will probably not be exactly 0. If the two device curves are slightly askew, then the two currents will not be identical. Biasing of this type is very popular in operational amplifiers. Another use for current mirrors is in the application of active loads. Instead of using simple resistors for the collector loads, a current mirror may be used instead. The current mirror active load produces a very high internal impedance, thus contributing to a very high differential gain.

In effect, by using a constant current source in the collectors, all AC current is forced into the following stage. You may also note that the number of resistors used in the circuit has decreased considerably.

This means that there are four variations on the theme: Differential also called dual- or double-ended input, differential output. Differential input, single-ended output. Single-ended input, differential output. Single-ended input, single-ended output.

Differential amplifier

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. Connect and share knowledge within a single location that is structured and easy to search. I'm trying to wrap my head around the workings of a differential amplifier when a current mirror is used as an 'active load'. With reference to this Youtube video leecture: here. If we now replace the load resistors with the current mirror where I think I'm misunderstanding some steps :. Now, with resistor loads the current was split such that it would add to the 10mA of the tail, with more on one side and less on the other.

Design of Differential Amplifier Using Current Mirror Load in 90 nm CMOS Technology

Guide to the study of. Read the Instructions to know how you can better use this work. Know how it is organized and which navigation tools are available. See how you can complement the study with the simulation of some of the circuits presented here. See the table of contents of this work. The table is organized through a pop down menu revealed when you place the cursor over the titles. Through the Index you can directly access each one of the sections and exercises of this work. The main text of this work is enhanced with several complementary texts, in order to help the reader about matters not directly studied here. These are matters which are supposed to be studied before or later.

US4194166A - Differential amplifier with a current mirror circuit - Google Patents

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US7545216B2 - Amplifier with current mirror bias adjust - Google Patents

Include Synonyms Include Dead terms. Peer reviewed Direct link. A differential amplifier composed of an emitter-coupled pair is useful as an example in lecture presentations and laboratory experiments in electronic circuit analysis courses. However, in an active circuit with zero input load V[subscript id], both laboratory measurements and PSPICE and LTspice simulation results for the output voltage V[subscript o] are considerably lower than one base emitter unit V[subscript BE on ] below the supply voltage V[subscript CC], as predicted by a textbook derivation. Modification of the derivation to include the p-n-p transistor base currents and the current gain beta[subscript P] and the supply voltage and specifications for the four transistors provide equations that predict results for V[subscript o] that are consistent with laboratory experience and computer simulations.

Oh no, there's been an error

Year of fee payment : 4. Year of fee payment : 8. Effective date : An amplifier includes a differential amplifier stage, a voltage amplification stage and a power output stage. The bias level of the output stage is proportional to current through the voltage amplification stage. The voltage amplification stage includes a current mirror whereby controlled current through a first leg of the current mirror controls current through the remaining, second leg, which, in turn, determines the bias level of the power output stage. Control circuitry senses a parameter of the amplifier, such as DC bias or input signal level to generate a control signal which is applied to the first leg of the current mirror to thereby control bias level of the power output stage. The amplifier is comprised of three stages:.

I have seen current mirrors being used as current source in differential stage of an amplifier design. The reason given is that the current mirror has a very high output impedance and behaves like an active load. I am not sure what this means. Why do we need to use a current mirror in differential stage of an op-amp?

Circuits and Systems Vol. The linearity, output impedance, bandwidth and accuracy are the most parameters to determine the performance of the current mirror. Here a comparison of two architectures based on same architecture of the amplifier is presented. This comparison includes: linearity, output impedance, bandwidth and accuracy.

Skip to Main Content. A not-for-profit organization, IEEE is the world's largest technical professional organization dedicated to advancing technology for the benefit of humanity. Use of this web site signifies your agreement to the terms and conditions. CMOS analog amplifier circuit sizing using opposition based harmony search algorithm Abstract: An optimum design of analog CMOS differential amplifier Diff-Amp with current mirror load has been presented in this paper. The novel Harmony Search HS algorithm is selected as the parent and the opposition based approach is employed to it with an intention to exhibit accelerated near-global convergence profile. This causes faster convergence profile.

In a differential amplifier having a current mirror circuit, the improvement where the collectors of a pair of transistors comprising the current mirror circuit are respectively connected to the bases of a pair of transistors of polarity reverse that of the current mirror transistors, the collectors of the reversed polarity transistors are connected to a power supply, the emitters of the reversed polarity transistors are connected to corresponding active elements of the differential amplifier, and resistors are respectively connected between the bases and emitters of the reversed polarity transistors. This invention relates to a differential amplifier used as a first stage amplifier or the like of an amplifying device in which a current mirror circuit for increasing load impedance is employed. Conventionally, this type of differential amplifier with a current mirror circuit is as shown in FIG.