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Differential amplifier circuits

Differential amplifier circuits

You are required to design two differential amplifier circuits which will convert input signals produced by a sensor in the ranges given below to the required output ranges.
a) Input range 0-1.Y volts to output range 0-10 volts
b) Input range 0-9.Z volts to output range 1-5 volts

Values Y and Z are based on the last two digits of your student number, so if for example, your student number was 123456789 then the required input ranges would be 0 – 1.8 V and 0 – 9.9 V respectively.

The last two digits: 53

Your task is to: –
1. design each circuit, documenting all design calculations.
2. configure simulations of each circuit, using these to verify and demonstrate the correct operation of your circuits.
3. identify practical components which could be used in the manufacture of each circuit, including realistic component values and commercially available electronic devices.
4. produce a full parts list and sample costings.
5. write a brief lab report containing evidence of completion of all tasks above, plus a discussion of your main findings.

Marking Scheme
a) Circuit design and calculations (20 marks)
b) Verification of correct operation – e.g. simulation (20 marks)
c) Component identification, parts list and costing (20 marks)
d) Written report (20 marks)
e) Discussion of findings (20 marks)

Hints and Tips
The differential amplifier circuit is discussed in Week 5 notes. There are worked examples similar to those given above in notes from Weeks 5 and 6.

The output voltage of the above differential amplifier is given by: –

𝑉 = 𝑅2 (𝑉 − 𝑉 )

𝑂𝑢𝑡 𝑅1 2 1

A recommended Proteus test circuit is given below, which you will need to create.


1) Key component names in the above simulation are “RES” – generic resistor and “OPAMP” – generic opamp. DC voltmeters are available from the Virtual Instruments section of the toolbar at the left side of the screen. Similarly, Power and Ground connectors are available from the Terminals section of the toolbar, at the left.
2) Proteus does not allow multiple components with the same name, hence the use of R1A, R1B, R2A and R2B in place of R1 and R2 respectively.
3) Input voltages V1 and V2 may be inputted in various ways. Power pins have been used in the above example. The voltage is set by entering the voltage as a text string. Examples include “+12V”, “-1V”, “+0V” (a leading plus or minus is always needed).

differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs.[1] It is an analog circuit with two inputs \displaystyle \scriptstyle V_\textin^-\scriptstyle V_\textin^- and \displaystyle \scriptstyle V_\textin^+\scriptstyle V_\textin^+ and one output \displaystyle \scriptstyle V_\textout\scriptstyle V_\textout in which the output is ideally proportional to the difference between the two voltages

\displaystyle V_\textout=A(V_\textin^+-V_\textin^-)V_\textout=A(V_\textin^+-V_\textin^-) where \displaystyle \scriptstyle A\scriptstyle A is the gain of the amplifier. Single amplifiers are usually implemented by either adding the appropriate feedback resistors to a standard op-amp, or with a dedicated integrated circuit containing internal feedback resistors. It is also a common sub-component of larger integrated circuits handling analog signals. Modern differential amplifiers are usually implemented with a basic two-transistor circuit called a “long-tailed” pair or differential pair. This circuit was originally implemented using a pair of vacuum tubes. The circuit works the same way for all three-terminal devices with current gain. The “long tail” resistor circuit bias points are largely determined by Ohm’s Law and less so by active component characteristics.

The long-tailed combine was made from earlier knowledge of push-pull circuit tactics and way of measuring bridges.[2] A young circuit which closely is similar to an extensive-tailed combine was authored by British neurologist Bryan Matthews in 1934,[3] plus it looks probable this was intended to be a real long-tailed combine but was released by using a attracting mistake. The very first definite extended-tailed match circuit presents itself in the patent sent in by Alan Blumlein in 1936.[4] In the end from the 1930s the topology was well known along with been described by a variety of authors which includes Frank Offner (1937),[5] Otto Schmitt (1937)[6] and Jan Friedrich Toennies (1938) [7] and it also was particularly utilized for recognition and measurement of biological impulses.[8]

The lengthy-tailed pair was very successfully used in early British processing, most notably the Initial ACE model and descendants,[nb 1] Maurice Wilkes’ EDSAC, and probably other folks produced by people who dealt with Blumlein or his friends. The very long-tailed combine has numerous positive features if applied as being a swap: mainly safe from pipe (transistor) versions (of wonderful relevance when devices contained one thousand pipes or even more), substantial acquire, acquire balance, higher enter impedance, medium sized/low output impedance, excellent clipper (having a not-too-long tail), non-inverting (EDSAC contained no inverters!) and huge productivity voltage swings. One downside would be that the result voltage golf swing (typically ±10–20 V) was imposed upon a high DC voltage (200 V or more), needing attention in indicate coupling, normally some kind of wide-music group DC coupling. A lot of personal computers on this time attempted to avert this issue by using only AC-paired pulse logic, which created them very large and overly sophisticated (ENIAC: 18,000 tubes for any 20 digit calculator) or untrustworthy. DC-coupled circuitry took over as the tradition after the very first era of vacuum tube pcs. The differential set can be used as an amplifier using a individual-ended insight if one of the inputs is grounded or fixed to your guide voltage (normally, the other collector is utilized as being a individual-finished productivity) This layout can be considered to be cascaded typical-collector and common-bottom levels or like a buffered common-foundation point.[nb 3]

The emitter-coupled amplifier is compensated for temperature drifts, VBE is cancelled, and the Miller result and transistor saturation are avoided. For this reason it is actually employed to develop emitter-combined amplifiers (preventing Miller result), stage splitter circuits (acquiring two inverse voltages), ECL gateways and changes (preventing transistor saturation), and many others.

Operations To describe the circuit functioning, four specific settings are isolated below despite the fact that, in reality, some of them act simultaneously as well as their results are superimposed.

Biasing In contrast with traditional amplifying steps that are biased through the area in the basic (and they also are highly β-based), the differential match is directly biased from your aspect of your emitters by sinking/injecting the whole quiescent current. The range bad feedback (the emitter damage) definitely makes the transistors act as voltage stabilizers it causes those to modify their VBE voltages (bottom currents) to move the quiescent existing through their collector-emitter junctions.[nb 4] So, due to adverse opinions, the quiescent existing depends only slightly in the transistor’s β.

The biasing base currents found it necessary to evoke the quiescent collector currents usually come from the terrain, move through the insight places and enter into the bases. So, the places need to be galvanic (DC) to guarantee routes for your biasing present and lower resistive enough not to produce considerable voltage declines across them. Usually, extra DC elements should be linked between the bases and the ground (or perhaps the positive power source).

Common method In frequent method (the two insight voltages improvement in exactly the same recommendations), both voltage (emitter) readers cooperate with one another cooperating in the frequent higher-resistive emitter load (the “long-tail”). All of them with each other increase or decrease the voltage of your typical emitter level (figuratively talking, they with each other “pull-up” or “draw down” it in order that it movements). In addition, the powerful stress “aids” them by changing its instant ohmic opposition inside the identical course since the enter voltages (it improves as soon as the voltage improves and the other way around.) thus retaining up continual complete opposition between your two source side rails. There is a whole (completely) adverse responses the two input bottom voltages as well as the emitter voltage alter simultaneously as the collector currents and the total current usually do not alter. Because of this, the production collector voltages do not modify too.

Differential mode Normal. In differential mode (the 2 feedback voltages alteration of opposing instructions), the 2 voltage (emitter) followers oppose each other—while one of these attempts to boost the voltage from the common emitter level, another tries to lower it (figuratively communicating, one of these “pulls up” the normal position even though the other “pulls straight down” it so that it continues to be immovable) and viceversa. So, the common level will not modify its voltage it behaves such as a internet soil by using a magnitude determined by the common-mode feedback voltages. Our prime-amount of resistance emitter aspect will not play any role—it is shunted by the other reduced-resistance emitter follower. There is no negative feedback, since the emitter voltage does not change at all when the input base voltages change. There is virtually no bad opinions, considering the emitter voltage is not really likely to modify at all as soon as the input groundwork voltages alter. Both the transistors mutually ground their emitters so, although they are typical-collector steps, they really serve as typical-emitter phases with maximum gain. Bias balance and self-sufficiency from variations in device parameters might be increased by unfavorable feedback introduced via cathode/emitter resistors with relatively modest resistances.

Overdriven. In the event the enter differential voltage alterations significantly (over regarding a hundred millivolts), the transistor driven through the reduced insight voltage transforms off along with its collector voltage gets to the optimistic source rail. At higher overdrive the basic-emitter junction receives reversed. The other transistor (powered through the greater input voltage) brings all the present. In the event the resistor with the collector is comparatively big, the transistor will saturate. With relatively little collector resistor and reasonable overdrive, the emitter can certainly still adhere to the insight sign without saturation. This mode is used in differential switches and ECL gates.

Breakdown. In case the feedback voltage persists increasing and is higher than the foundation-emitter breaking down voltage, the foundation-emitter junction of the transistor motivated by the reduced feedback voltage reduces. In case the input sources are low resistive, an unlimited present will stream directly with the “diode link” involving the two input options and can damage them.

In popular method, the emitter voltage practices the enter voltage variants you will find a full negative feedback and the acquire is minimum. In differential function, the emitter voltage is resolved (comparable to the fast frequent feedback voltage) there is no bad feedback and the acquire is highest.