3: BASIC CIRCUITS USING OP AMPS:
Op amps can be used in many different configurations, but the simple circuits described here cover most applications.
Unity gain buffer, inverting and non-inverting amplifier, difference amplifier
If you feel some of this is getting too technical you may wish to skip any bits in italics.
Unity gain buffer.
A "unity gain buffer" gives an output voltage that is the same as the input voltage
The important feature of this circuit is that it takes almost no current from the input. It can be used to measure voltages without placing a load on the circuit being measured. Lets look at how it works.
The unity gain buffer is very easy to analyse, based just on the characteristics of an ideal op amp.
As the amplifier has infinite gain
from #1: - the difference voltage between the + and - inputs must be zero.
R1 and R2 just provide protection to the amplifier inputs. "No" current flows through them.
so Vo = Va
Non-inverting amplifier
Like the unity gain buffer, this circuit does not load the input - but it does provide a precise amount of voltage gain, set by resistors R1 & R2.
Vout = Vin (R2 + R1 / R1) or Vout / Vin = 1 + (R2 / R1)
The input resistance is VERY HIGH (ideally "infinite")
Remember, for any finite voltage at the output the difference voltage at the input must be zero.
Lets analyse this circuit just using the ideal op amp characteristics and you will see how easy it is. We'll use real numbers.
Suppose Vo = 10V and R1=2k, R2 = 18k
R1 + R2 = 20k so a current of 10V / 20k = 0.5mA flows through the resistor chain.
NONE of that current flows into the inverting input. (2:) ( I2 = I1 )
so the voltage across R1 is 0.5mA * 2k = 1V
From 1: Vb = Va .. therefore the voltage gain Vo / Va = 10V / 1V = 10.
Inverting amplifier
This circuit produces an output voltage that is the INVERSE of the input voltage. For example a negative input voltage will result in a positive voltage at the output. This type of amplifier circuit is used in our first example (Reply 4).
Vout = - Vin R2 / R1 or Vout / Vin = - R2/R1 ( Note: this formula is not the same as for the non-inverting amplifier)
The input resistance Rin is just R1. However with modern op amps we can use large value resistors.
Once again, Remember (1:) the difference voltage between the + and - inputs must be zero: so the junction of R1 and R2 is at 0V. (we call this a "virtual earth")
Vo = I2 R2
Also (2:) no current flows into the input terminals - so I1 = I2
Va = - I1 R1 = - I2 R1.
Vo / Va = I2 R2 / - I2 R1 = - R2 / R1
Important note: Unlike the circuits of Fig 1 and Fig 2 this circuit involves a current flowing through the input circuit, so the gain WILL depend on the source resistance. The usual way to avoid this is to use a unity gain buffer at the input.
Difference amplifier
This simply measures the DIFFERENCE between the two inputs and multiplies it by a gain factor. A difference amplifier is used in our second example (Reply 8)
Vo = (Vb - Va) (R2/R1) ;
If R2 = R1 the gain is set to 1 and the output voltage is just the difference between the two input voltages.
By choosing different values we can add a bit of gain;
If R2 = 10M and R1 = 1M gives a gain of ten.
Now the input resistance for this circuit is just R1 + R2, so if you dont want to load the circuit you are measuring R1 and R2 need to be large - and you will need to choose an op amp with a * low input bias current.
(more on this later - reply 7): basic circuits
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