Higher Electricity- Operational Amplifiers
What is the usual supply voltage for an op amp?
+15V/-15V
What will you notice about the output from a differential op amp?
A. The output is NOT inverted - if the input is positive the output is positive and vice versa. B The output is multiplied by (Rf/R1) ₁until the op amp becomes saturated.
What three things must you remember about the output of an inverting op amp?
A - The output signal is negative when the input is positive, and vice versa B - the output is multiplied by the ratio Rf/R₁ C - the frequency is the same throughout
What does an op amp actually look like?
An op-amp chip.
What is an op-amp?
An operational amplifer is an electronic device that is used to increase the voltage of a signal. It can be connected in several different ways.
What are the characteristics of an ideal op amp?
Both inputs are * at the same potential (zero) * have infinite resistance (no input current)
What is the function of the resistor R₂?
It provides negative feedback from the output. this helps control the gain of the amplifier. It is usually labelled Rf. (f for feedback)
What are the op amp modes used in Higher?
Inverting mode (two resistors) and Differential Mode (four resistors)
Why is a differential op amp circuit useful as the next stage in a wheatstone bridge circuit?
It can amplify the difference between the inputs sufficiently to drive a third part of the circuit. Here when the voltage rises above 0.7V the transistor switches on and the heater goes on.
What is an ideal op-amp?
It has infinite input resistance - the input current is zero. Both inputs are at the same potential - usually zero.
What happens to the non-inverting input in inverting mode?
It is tied to 0V (i.e. it is not used)
What effect will LDR and Thermistors have on a differential circuit?
Light decreases resistance, temperature up, resistance down. As light level increases, Rldr increases and Vldr increases.
Is it correct to say that the voltage has become saturated in an op amp?
NO - you will not get any marks. You must say that the op amp is saturated.
How do you calculate the voltage at V₁and V₂if the input is a wheatstone bridge?
Remember the wheatstone bridge is just two potential dividers side by side. The voltage across the lower resistor is the lower resistance over the total resistance, times the supply voltage.
How are all the resistors in the differential op amp related?
Rf/R₁= R₃/R₂ In effect you will just have to deal with Rf/R₁
Draw a diagram of an op amp in inverting mode.
R₂is usually labelled Rf.
How can the two modes be identified?
Simples - THe inverting op amp has two resistors, the differential op amp has four resistors. Not so simples - The non-inverting input is tied to zero volts in the inverting amplifier (giving just one input)
What circuit could be used as the input to a differential op amp?
THe Wheatstone bridge.
Which input is the non-inverting input?
THe one with the plus sign
Why does an ideal op-amp have both inputs at the same potential?
The amplifier would amplify any small difference and affect the output signal.
What is meant by the gain of a circuit?
The gain is the Output voltage/ input voltage OR The gain is the Output power /Input power NOTE - there is NO unit
Which input is the one with the minus sign?
The inverting input
What input is used as V₁?
The inverting input (the one with the minus)
What do we mean by the mode of an op amp?
The mode of an op amp is the way it is connected. For Higher you need to know two modes.
What input is used as V₂?
The non-inverting input (the one with the plus)
Which input is the one with the plus sign?
The non-inverting input.
Which input is the inverting input?
The one with the negative sign
Why is the output voltage limited to ±15V?
The op amp becomes saturated, so cannot go beyond the power supply voltage. OR The output voltage cannot exceed the power supply voltage.
Why is a Wheatstone bridge circuit a good input for a differential amplifier?
The op amp output is the difference between the two inputs (V₂-V₁) multiplied by Rf/R. The difference between the inputs is what would show on the voltmeter in a traditional wheatstone bridge circuit.
What happens to the output of an inverting op amp if the gain is increased to the point where it saturates almost immediately and the input is a sine wave?
The output becomes an inverted square wave with Vmax = ±Vs (or 85% Vs)
What will you observe if Rf = 10kΩ and R₁= 5kΩ?
The output signal will be twice as big as the input signal (Rf/R₁= 10/5 = 2) and inverted. Remember to check the power supply of the op-amp in case it becomes saturated.
If the input to an inverting op amp is a sine wave and it is operating just at the point of saturation - what is the effect on the output voltage?
The output voltage will still be a sine wave, but the tops of each wave will be clipped (flattened) as well as it being inverted.
What are the connections on an op amp?
There are two inputs - the inverting input (the one with the minus) and the non-inverting input (the one with the plus), and one output. It also has two power supply rails + Vs and -Vs
What are the units for gain?
There is NO unit. You will lose half a mark if you give a unit. In either case you have divided the same unit top and bottom, so it ends up cancelling out.
What is meant by 'saturation'?
This is when the output voltage, by calculation, would be greater than the power supply voltage of the amplifier. This cannot happen, so you are limited to ±Vs (or 85% of this).
What is negative feedback?
This is where a small part of the output signal is fed back into the input, but it reduces the input. This helps control the output. This is used in operational amplifer circuits.
What is positive feedback?
This is where part of the output signal is fed back into the input. The output signal keeps on getting bigger until it causes distortion - like someone walking in front of a loudspeaker with a microphone.
Why does an ideal op-amp have infinite input resistance?
This means there is no current flowing into the op amp.
What is a differential op amp used for?
To amplify the difference between two input signals.
What is the role of the power supply rails?
To provide the energy needed to amplify the signal.
What are typical input voltages for an op amp?
Usually +15V/-15V, but also +12V/-12V or +9V/-9V If no voltage is shown you should assume ±15V
What is the equation for the output voltage of a differential op amp?
Vo = (V₂-V₁) Rf/R₁
What is the equation for the gain of an inverting op amp?
Vo/Vi = - Rf/R₁
What is the equation for V₀ in an inverting op amp circuit?
V₀ = - (Rf / R₁) Vi
What is the maximum output voltage of an op amp in practice?
± 85% Vs
What is the theoretical maximum output voltage of an op amp which has Vs at ±12V
±12V
