Fundamental Physics/Electricity/Electric circuits/Electric Circuits

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Electric Circuits [edit]

Resistor Circuit[edit]

Ohms law voltage source.svg
Entities Symbol Mathematical Formula Unit
Voltage Volt . v
Current Ampere . a
Resistance Ohom . Ω
Conductance 1/Ω
Power provided Watt . W
Power loss W
Power transmitted W

RL Circuit[edit]

RL Circuit refers to a circuit having combination of resistance(s) and inductor(s). They are commonly used in chokes of luminescent tubes. In an A.C. circuit, inductors helps in reducing voltage, without the loss of energy. Due to the inductive reactance, the higher the AC frequency, the greater the impeadence of the inductor. Under DC conditions, an inductor acts as a static resistance.

Like RC circuit , with one resistor and one coil can be connected to form a low pass filter or a high pass filter. A high-pass filter allows frequencies above the cut-off frequency to pass, while a low-pass filter allows frequencies beneath the cut-off frequency to pass. The arrangement of the resistor and the capacitor is what determines their behaviour.

Note that at a particular frequencly, called the cut-off frequency, the Inductive Reactance is equal to the Resitance value. (There is also an associated phase shift of 45 degrees.)

Substituting we then have:

The cut-off frequency, defined as the frequency at which the signal power is attenuated by 50% (or 3.01 dB), is a function of the resistive and capacitive values. We can rearrange the above formula to solve for as follows:

RL Series[edit]

A circuit of 2 component a resistor and an inductor connected in series

High pass filter[edit]

When the inductor is in parallel with the load while the resistor is in series with the inductor and load, this creates a high pass filter.

Series-RL.svg

High pass filter has a transfer function

Frequency response of High pass filter

Cut off frequency, , frequency at which

Low pass filter[edit]

When the resistor is in parallel with the load while the inductor is in series with the resistor and load, a low pass filter is created.


Low pass filter has a transfer function

Frequency response of Low pass filter

Cut off frequency, , frequency at which

A single RL circuit creates a filter with a 20.0 dB/decade, or 6.02 dB/octave, slope.

RC Circuits[edit]

RC circuits are circuits that contain a resistor and a capacitor. These circuits are primarily used as frequency filters. There are two basic arrangements: high-pass and low-pass. A high-pass filter allows frequencies above the cut-off frequency to pass, while a low-pass filter allows frequencies beneath the cut-off frequency to pass. The arrangement of the resistor and the capacitor is what determines their behaviour.

Note that at a particular frequencly, called the cut-off frequency, the Capactive Reactance is equal to the Resistance value. (There is also an associated phase shift of 45 degrees.)

Substituting

we then have:

The cut-off frequency, defined as the frequency at which the signal power is attenuated by 50% (or 3.01 dB), is a function of the resistive and capacitive values. We can rearrange the above formula to solve for as follows:

RC series[edit]

A circuit of 2 component a resistor and a capacitor connected in series


Low pass filter[edit]

When the capacitor is in parallel with the load while the resistor is in series with the capacitor and load, this creates a low pass filter.

Low pass filter.png

Low pass filter has a transfer function

Frequency response of Low pass filter

Cut off frequency, , frequency at which

High pass filter[edit]

When the resistor is in parallel with the load and the capacitor is in series with the resistor, a high pass filter is created.

High pass filter.png

High pass filter has a transfer function

Frequency response of High pass filter

Cut off frequency, , frequency at which

A single RC circuit creates a filter with a 20.0 dB/decade, or 6.02 dB/octave, slope.

RLC Circuit[edit]

A circuit of 3 component a resistor and a capacitor and an inductor connected in series

Circuit at equilibrium[edit]

Lapalce transform yield

Solution to the above equation

  • 1 real root
  • 2 real roots
  • 2 complex roots

Circuit at resonant[edit]

LC Circuit[edit]

A circuit of 2 component a capacitor and an inductor connected in series

Circuit at equilibrium[edit]

Circuit at Resonant[edit]

Diode's circuits[edit]

An electronics switch[edit]

Diode is switch On when
Diode is switch Off when

Rectifier[edit]

Transistor's circuits[edit]

With the right connection, by connecting transistor with resistor(s) Transistor can acts as

An electronics amplifier[edit]

Current amplifier[edit]

Provided that, transistor should be turned ON with biased voltage at the base must be greater than diode's break over voltage

.
Non inverting voltage amplifier[edit]

With

and . . Transistor acts as Switch OFF (open circuit)
and . . Transistor acts as Switch ON (close circuit)


With

and . . Transistor acts as Non inverting amplifier
Inverting voltage amplifier[edit]


With

and . . Transistor acts as Switch ON (close circuit)
and . . Transistor acts as Switch OFF (open circuit)


With

and . . Transistor acts as Inverting amplifier

An electronics switch[edit]

Transistor is switch On when
Transistor is switch Off when

A buffer[edit]

Operational amplifier's circuits[edit]

Amplifiers[edit]

We begin these examples with that of the differential amplifier, from which many of the other applications can be derived, including the inverting, non-inverting, and summing amplifier, the voltage follower, integrator, differentiator, and gyrator.

Differential amplifier (difference amplifier)[edit]

Op-Amp Differential Amplifier.svg

Main article: Differential amplifier

Amplifies the difference in voltage between its inputs.

The name "differential amplifier" must not be confused with the "differentiator," which is also shown on this page.
The "instrumentation amplifier," which is also shown on this page, is a modification of the differential amplifier that also provides high input impedance.

The circuit shown computes the difference of two voltages, multiplied by some gain factor. The output voltage:

Or, expressed as a function of the common mode input Vcom and difference input Vdif

the output voltage is

In order for this circuit to produce a signal proportional to the voltage difference of the input terminals, the coefficient of the Vcom term (the common-mode gain) must be zero, or

With this constraint If you think of the left-hand side of the relation as the closed-loop gain of the inverting input, and the right-hand side as the gain of the non-inverting input, then matching these two quantities provides an output insensitive to the common-mode voltage of and .</ref> in place, the common-mode rejection ratio of this circuit is infinitely large, and the output

where the simple expression Rf / R1 represents the closed-loop gain of the differential amplifier.

The special case when the closed-loop gain is unity is a differential follower, with:

Inverting amplifier[edit]

Op-Amp Inverting Amplifier.svg

An inverting amplifier is a special case of the differential amplifier in which that circuit's non-inverting input V2 is grounded, and inverting input V1 is identified with Vin above. The closed-loop gain is Rf / Rin, hence

.

The simplified circuit above is like the differential amplifier in the limit of R2 and Rg very small. In this case, though, the circuit will be susceptible to input bias current drift because of the mismatch between Rf and Rin.

To intuitively see the gain equation above, calculate the current in Rin:

then recall that this same current must be passing through Rf, therefore (because V = V+ = 0):

A mechanical analogy is a seesaw, with the V node (between Rin and Rf) as the fulcrum, at ground potential. Vin is at a length Rin from the fulcrum; Vout is at a length Rf. When Vin descends "below ground", the output Vout rises proportionately to balance the seesaw, and vice versa.


Non-inverting amplifier[edit]

Op-Amp Non-Inverting Amplifier.svg

A non-inverting amplifier is a special case of the differential amplifier in which that circuit's inverting input V1 is grounded, and non-inverting input V2 is identified with Vin above, with R1R2. Referring to the circuit immediately above,

.

To intuitively see this gain equation, use the virtual ground technique to calculate the current in resistor R1:

then recall that this same current must be passing through R2, therefore:

Unlike the inverting amplifier, a non-inverting amplifier cannot have a gain of less than 1.

A mechanical analogy is a class-2 lever, with one terminal of R1 as the fulcrum, at ground potential. Vin is at a length R1 from the fulcrum; Vout is at a length R2 further along. When Vin ascends "above ground", the output Vout rises proportionately with the lever.

The input impedance of the simplified non-inverting amplifier is high, of order Rdif × AOL times the closed-loop gain, where Rdif is the op amp's input impedance to differential signals, and AOL is the open-loop voltage gain of the op amp; in the case of the ideal op amp, with AOL infinite and Rdif infinite, the input impedance is infinite. In this case, though, the circuit will be susceptible to input bias current drift because of the mismatch between the impedances driving the V+ and V op amp inputs.


Voltage follower (unity buffer amplifier)[edit]

Op-Amp Unity-Gain Buffer.svg

Used as a buffer amplifier to eliminate loading effects (e.g., connecting a device with a high source impedance to a device with a low input impedance).

(realistically, the differential input impedance of the op-amp itself, 1 MΩ to 1 TΩ)

Due to the strong (i.e., unity gain) feedback and certain non-ideal characteristics of real operational amplifiers, this feedback system is prone to have poor stability margins. Consequently, the system may be unstable when connected to sufficiently capacitive loads. In these cases, a lag compensation network (e.g., connecting the load to the voltage follower through a resistor) can be used to restore stability. The manufacturer data sheet for the operational amplifier may provide guidance for the selection of components in external compensation networks. Alternatively, another operational amplifier can be chosen that has more appropriate internal compensation.


Summing amplifier[edit]
Op-Amp Summing Amplifier.svg

A summing amplifier sums several (weighted) voltages:

  • When , and independent
  • When
  • Output is inverted
  • Input impedance of the nth input is ( is a virtual ground)
Instrumentation amplifier[edit]

Op-Amp Instrumentation Amplifier.svg

Main article: Instrumentation amplifier

Combines very high input impedance, high common-mode rejection, low DC offset, and other properties used in making very accurate, low-noise measurements

Oscillators[edit]

Wien bridge oscillator[edit]

Wien bridge classic osc.svg

Produces a very low distortion sine wave. Uses negative temperature compensation in the form of a light bulb or diode.

Comparator[edit]

Op-Amp Comparator.svg


An operational amplifier can, if necessary, be forced to act as a comparator. The smallest difference between the input voltages will be amplified enormously, causing the output to swing to nearly the supply voltage. However, it is usually better to use a dedicated comparator for this purpose, as its output has a higher slew rate and can reach either power supply rail. Some op-amps have clamping diodes on the input that prevent use as a comparator.

Integration and differentiation[edit]

Inverting integrator[edit]

The integrator is mostly used in analog computers, analog-to-digital converters and wave-shaping circuits. Op-Amp Integrating Amplifier.svg

Integrates (and inverts) the input signal Vin(t) over a time interval t, t0 < t < t1, yielding an output voltage at time t = t1 of

where Vout(t0) represents the output voltage of the circuit at time t = t0. This is the same as saying that the output voltage changes over time t0 < t < t1 by an amount proportional to the time integral of the input voltage:

This circuit can be viewed as a low-pass electronic filter, one with a single pole at DC (i.e., where ) and with gain.


A slightly more complex circuit can ameliorate the second two problems, and in some cases, the first as well. 100pxl

Here, the feedback resistor Rf provides a discharge path for capacitor Cf, while the series resistor at the non-inverting input Rn, when of the correct value, alleviates input bias current and common-mode problems. That value is the parallel resistance of Ri and Rf, or using the shorthand notation ||:

The relationship between input signal and output signal is now:


Inverting differentiator[edit]

Op-Amp Differentiating Amplifier.svg

Differentiates the (inverted) signal over time.

  • This can also be viewed as a high-pass electronic filter. It is a filter with a single zero at DC (i.e., where angular frequency  radians) and gain. The high-pass characteristics of a differentiating amplifier (i.e., the low-frequency zero) can lead to stability challenges when the circuit is used in an analog servo loop (e.g., in a PID controller with a significant derivative gain). In particular, as a root locus analysis would show, increasing feedback gain will drive a closed-loop pole toward marginal stability at the DC zero introduced by the differentiator.

Synthetic elements[edit]

Inductance gyrator[edit]

Op-Amp Gyrator.svg

Main article: Gyrator

Simulates an inductor (i.e., provides inductance without the use of a possibly costly inductor). The circuit exploits the fact that the current flowing through a capacitor behaves through time as the voltage across an inductor. The capacitor used in this circuit is smaller than the inductor it simulates and its capacitance is less subject to changes in value due to environmental changes.

This circuit is unsuitable for applications relying on the back EMF property of an inductor as this will be limited in a gyrator circuit to the voltage supplies of the op-amp.

Negative impedance converter (NIC)[edit]

Op-Amp Negative Impedance Converter.svg

Creates a resistor having a negative value for any signal generator.

In this case, the ratio between the input voltage and the input current (thus the input resistance) is given by:

In general, the components , , and need not be resistors; they can be any component that can be described with an impedance.

Non-linear[edit]

Precision rectifier[edit]

Op-Amp Precision Rectifier.svg


The voltage drop VF across the forward biased diode in the circuit of a passive rectifier is undesired. In this active version, the problem is solved by connecting the diode in the negative feedback loop. The op-amp compares the output voltage across the load with the input voltage and increases its own output voltage with the value of VF. As a result, the voltage drop VF is compensated and the circuit behaves very nearly as an ideal (super) diode with VF = 0 V.

The circuit has speed limitations at high frequency because of the slow negative feedback and due to the low slew rate of many non-ideal op-amps.

Logarithmic output[edit]

Op-Amp Logarithmic Amplifier.svg

  • The relationship between the input voltage Vin and the output voltage Vout is given by:
where IS is the saturation current and VT is the thermal voltage.
  • If the operational amplifier is considered ideal, the inverting input pin is virtually grounded, so the current flowing into the resistor from the source (and thus through the diode to the output, since the op-amp inputs draw no current) is:
where ID is the current through the diode. As known, the relationship between the current and the voltage for a diode is:
This, when the voltage is greater than zero, can be approximated by:
Putting these two formulae together and considering that the output voltage is the negative of the voltage across the diode (Vout = −VD), the relationship is proven.

This implementation does not consider temperature stability and other non-ideal effects.


Exponential output[edit]

Op-Amp Exponential Amplifier.svg

  • The relationship between the input voltage and the output voltage is given by:

where is the saturation current and is the thermal voltage.

  • Considering the operational amplifier ideal, then the negative pin is virtually grounded, so the current through the diode is given by:

when the voltage is greater than zero, it can be approximated by:

The output voltage is given by: