Electronics SL – Option C.3 HL – Option F.5

Operational Amplifier An amplifier is a device that takes a signal and makes its bigger. – An mp3 player gives out a very small pd that you can hear with your headphones – If you connect it to a pair of speakers the sound can be amplified with a power source An operation amplifier (op-amp) can be made to do more than just amplify signals, based on how it is wired

Op-Amp An actual op amp is a small black rectangle called a chip – The chip has 8 ‘legs’ or connections but only 5 are used for our purposes – Inside the chip is a complicated circuit composed of transistors and resistors that we don’t need to be concerned with

Op Amp In the top diagram (how an op amp is represented in a circuit diagram); – The inputs are where the signal is fed into the amp – The output is where the amplified signal comes out – The +/- V is where the supply (power is connected) This is the energy that is used to amplify the signal – In a circuit diagram the V+ and V- are often omitted

The Ideal Op Amp An ideal operational amplifier has three primary characteristics – Infinite input impedance – Zero output impedance – Infinite gain

Input impedance This is essentially the resistance that would be measured across the input terminals – This means that the input current to the amplifier is negligible – The amplifier measures the difference between the two inputs

Output Impedance The output of an op amp is like a power source – It can be used to power loudspeakers, lightbulbs, etc. – The output impedance is equal to the internal resistance of the power source In an ideal op amp this is zero – To get a current to flow through a load resistor there must be a pd across it This is done by connecting the resistor to a wire at 0V. This is called an earth or ground

Open Loop Gain If a very small pd is applied between the inputs, a large potential is created on the output – The ratio: is called the open loop gain of the amplifier This value is typically in the range of 10 6 This means that if the pd between the input terminals were 0.01V, then the output potential would be V In an ideal op amp this is infinite but it is controlled by the supply voltage

Gain A lone op amp is not particularly useful for amplifying music – If the open loop gain is 10 6, once the signal goes above V, the output will be a constant 9V with a 9V supply – The amplifier must be able to produce a signal that is the same as the input but with a larger amplitude

Inverting Op Amp An op amp is said to be inverting if the + (non- inverting) input is grounded and the – (inverting) input is connected to a resistor and an input signal – R in is the input resistor – R f is the feedback resistor since it is feeding the output voltage back to the input

Inverting Op Amp When a voltage is applied to the inverting input (-) a voltage of the opposite polarity develops on the output – if V in is +, then V out is -; if V in is -, then V out is +

Inverting Op Amp Finding the Gain – The difference between V+ and V- must be small so we can say they are equal Since + is connected to Earth, The potential at a point (P) between R1 and R2 is therefore also 0V Applying Ohm’s Law to R1 and R2

Non-Inverting Op Amp An op amp is said to be non- inverting if the – (inverting) input is grounded and the + (non- inverting) input is connected to an input signal – The polarity of the voltage at the output is the same as that of the input (if V in is +, then V out is +; if V in is -, then V out is - )

Non-Inverting Op Amp The input signal is applied to the non-inverting (+) lead The amount the signal is amplified is determined by the value of resistors R out and R f – If the difference between + and – is more than 9μV, the output will be a constant 9V so we can say the two inputs are about the same – This means that the potential at point P is also V input

Non-Inverting Op Amp If the current through the resistors is I then we get To R out alone; The 2 currents are equal so

Comparator A comparator compares two potentials – If the potential on the + input is higher, then the output is V+ and if the – input is higher, then the output is V-

Comparator Fire Alarm – At normal room temp all resistances are equal so both inputs equal V and the output is 0 – If the temp increases then the resistance of the thermistor drops and the pd across it drops. This results in an increase in the potential at the – input which results in an output of 9 V and the bell will ring

Schmitt Trigger A type of comparator that switches to a high output when the input is above a given value (upper threshold) – Only switches to a low output when the input gets lower than another low value (lower threshold) – Schmitt triggers are often used for reshaping digital signals to remove noise

Schmitt Trigger