Practical Electronics/Operating amplifiers

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Intro[edit]

Op Amp is a short hand term for Operational Amplifier. An operational amplifier is a circuit component that amplifies the difference of two input voltages:

Vo = A (V2 - V1)

Op Amps are usually packaged as an 8-pin integrated circuit.

Operational Amplifier IC Chip
Pin Usage
1 Offset Null
2 Inverted Input
3 Non-Inverted Input
4 -V Supply
5 No use
6 Output
7 +V Supply
8 No use


Op Amp symbol op-amp

  • V+: non-inverting input
  • V: inverting input
  • Vout: output
  • VS+: positive power supply
  • VS−: negative power supply

Op amps amplify AC signal or AC Voltage better than a simple bipolar junction transistor.

Op Amp Functions[edit]

Voltage Difference Amplifier[edit]

From above

V0 = A (V2 - V1)

Voltage Comparator[edit]

V2 > V1 , V0 = +Vss
V2 < V1 , V0 = -Vss
V2 = V1 , V0 = 0

Inverting Amplifier[edit]

With one voltage is grounded

If V2 = 0 , V0 = -A V1 . Inverting Amplifier

Non-Inverting Amplifier[edit]

With one voltage is grounded

If V1 = 0 , V0 = A V2 . Non-Inverting Amplifier

Linear Configurations[edit]

Differential amplifier[edit]

Differential amplifier
 V_\mathrm{out} = V_2 \left( { \left( R_\mathrm{f} + R_1 \right) R_\mathrm{g} \over \left( R_\mathrm{g} + R_2 \right) R_1} \right) - V_1 \left( {R_\mathrm{f} \over R_1} \right)
  • Differential Z_\mathrm{in} (between the two input pins) = R_1 + R_2

Voltage Difference Amplifier[edit]

Whenever R_1 = R_2 and R_\mathrm{f} = R_\mathrm{g},

 V_\mathrm{out} = {R_\mathrm{f} \over R_1} \left( V_2 - V_1 \right)

Voltage Difference[edit]

When R_1 = R_\mathrm{f} and R_2 = R_\mathrm{g} (including previous conditions, so that R_1 = R_2 = R_\mathrm{f} = R_\mathrm{g}):

 V_\mathrm{out} =  V_2 - V_1 \,\!

Inverting Amplifier[edit]

Inverting amplifier
 V_\mathrm{out} = - V_\mathrm{in} \left( {R_f \over R_1} \right)

Inverting Amplification is dictated by the ratio of the two resistors

Non-Inverting Amplifier[edit]

Non-inverting amplifier
 V_\mathrm{out} = V_\mathrm{in} \left( 1 + {R_2 \over R_1} \right)

Non-Inverting Amplification is dictated by the ratio of the two resistors plus one

Voltage Follower[edit]

Voltage follower

From Non-Inverting Amplifier's formula. If the resistors has the same value of resistance then output voltage is exactly equal to the input voltage

 V_\mathrm{out} = V_\mathrm{in} \!\

From Inverting Amplifier's formula. If the resistors has the same value of resistance then output voltage is exactly equal to the input voltage and inverted

 V_\mathrm{out} = - V_\mathrm{in} \!\

Summing amplifier[edit]

Summing amplifier
 V_\mathrm{out} = - R_\mathrm{f} \left( { V_1 \over  R_1 } + { V_2 \over R_2 } + \cdots + {V_n \over R_n} \right)

When R_1 = R_2 = \cdots = R_n, and R_\mathrm{f} independent

 V_\mathrm{out} = - \left( {R_\mathrm{f} \over R_1} \right) (V_1 + V_2 + \cdots + V_n ) \!\

When R_1 = R_2 = \cdots = R_n = R_\mathrm{f}

 V_\mathrm{out} = - ( V_1 + V_2 + \cdots + V_n ) \!\

Integrator[edit]

Integrating amplifier

Integrates the (inverted) signal over time

 V_\mathrm{out} = \int_0^t - {V_\mathrm{in} \over RC} \, dt + V_\mathrm{initial}

(where V_\mathrm{in} and V_\mathrm{out} are functions of time, V_\mathrm{initial} is the output voltage of the integrator at time t = 0.)

Differentiator[edit]

Differentiating amplifier

Differentiates the (inverted) signal over time.

The name "differentiator" should not be confused with the "differential amplifier", also shown on this page.

V_\mathrm{out} = - RC \left( {dV_\mathrm{in} \over dt} \right)

(where V_\mathrm{in} and V_\mathrm{out} are functions of time)

Comparator[edit]

Comparator
  •  V_\mathrm{out} = \left\{\begin{matrix} V_\mathrm{S+} & V_1 > V_2 \\ V_\mathrm{S-} & V_1 < V_2 \end{matrix}\right.

Từ V0 = A (V2 - V1)

  • Vo = 0 khi V2 = V1
  • Vo > 0 khi V2 > V1
Vo = Vss
  • Vo < 0 khi V2 < V1
Vo = V-ss

When two input voltages equal. The output voltage is zero . When the two input voltages different and if one is greater than or less than the other

  1. Vo = Vss khi V2 > V1
  2. Vo = V-ss khi V2 < V1

Instrumentation amplifier[edit]

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

Schmitt trigger[edit]

Schmitt trigger

A comparator with hysteresis

Hysteresis from \frac{-R_1}{R_2}V_{sat} to \frac{R_1}{R_2}V_{sat}.

Gyrator[edit]

Inductance gyrator

A gyrator can transform impedances. Here a capacitor is changed into an inductor.

 L = R_\mathrm{L} R C

Zero level detector[edit]

Voltage divider reference

  • Zener sets reference voltage

Negative impedance converter (NIC)[edit]

Negative impedance converter


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:
R_\mathrm{in} = - R_3 \frac{R_1}{R_2}

Non-linear configurations[edit]

Rectifier[edit]

Super diode

Behaves like an ideal diode for the load, which is here represented by a generic resistor R_\mathrm{L}.

  • This basic configuration has some limitations. For more information and to know the configuration that is actually used, see the main article.

Peak detector[edit]

Peak detector

When the switch is closed, the output goes to zero volts. When the switch is opened for a certain time interval, the capacitor will charge to the maximum input voltage attained during that time interval.

The charging time of the capacitor must be much shorter than the period of the highest appreciable frequency component of the input voltage.

Logarithmic output[edit]

Logarithmic configuration
  • The relationship between the input voltage v_\mathrm{in} and the output voltage v_\mathrm{out} is given by:
v_\mathrm{out} = -V_{\gamma} \ln \left( \frac{v_\mathrm{in}}{I_\mathrm{S} \cdot R} \right)

where I_\mathrm{S} is the saturation current.

  • If the operational amplifier is considered ideal, the negative 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:
\frac{v_\mathrm{in}}{R} = I_\mathrm{R} = I_\mathrm{D}

where I_\mathrm{D} is the current through the diode. As known, the relationship between the current and the voltage for a diode is:

I_\mathrm{D} = I_\mathrm{S} \left( e^{\frac{V_\mathrm{D}}{V_{\gamma}}} - 1 \right)

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

I_\mathrm{D} \simeq I_\mathrm{S} e^{V_\mathrm{D} \over V_{\gamma}}

Putting these two formulae together and considering that the output voltage V_\mathrm{out} is the inverse of the voltage across the diode V_\mathrm{D}, the relationship is proven.

Note that this implementation does not consider temperature stability and other non-ideal effects.

Exponential output[edit]

Exponential configuration
  • The relationship between the input voltage v_\mathrm{in} and the output voltage v_\mathrm{out} is given by:
v_\mathrm{out} = - R I_\mathrm{S} e^{v_\mathrm{in} \over V_{\gamma}}

where I_\mathrm{S} is the saturation current.

  • Considering the operational amplifier ideal, then the negative pin is virtually grounded, so the current through the diode is given by:
I_\mathrm{D} = I_\mathrm{S} \left( e^{\frac{V_\mathrm{D}}{V_{\gamma}}} - 1 \right)

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

I_\mathrm{D} \simeq I_\mathrm{S} e^{V_\mathrm{D} \over V_{\gamma}}

The output voltage is given by:

v_\mathrm{out} = -R I_\mathrm{D}\,