Analogue Electronics Higher Physics Unit 2 Electricity And Electronics Introduction to Op-Amps

Introduction The heart of a modern computer, the CPU, is a digital device using the switching action of transistors to ‘make decisions’.

Introduction The heart of any hi-fi system, the amplifier, is an analogue device using the ‘voltage gain’ property of transistors to increase the input signal voltage (to drive a loudspeaker).

Analogue v Digital Digital devices operate in only 2 states (Low or High), while analogue devices operate over a range of voltagesDigital devices operate in only 2 states (Low or High), while analogue devices operate over a range of voltages The Operational Amplifier (Op-Amp) is an analogue device designed to increase the voltage of an input signal.The Operational Amplifier (Op-Amp) is an analogue device designed to increase the voltage of an input signal. ANALOGUE SIGNAL

The Op-Amp is usually in the form of an integrated circuit (i/c or microchip) It requires a positive and negative voltage supply, called a dual- rail supply

The Op-Amp has 2 input terminals. The inverting (-) terminal allows the input signal to be inverted as it is amplified. The non-inverting (+) terminal amplifies the input signal without inverting it.

The Ideal Op-Amp Is designed to have infinite input resistance, so that zero current flows into the amp itselfIs designed to have infinite input resistance, so that zero current flows into the amp itself Is designed so that both input terminals are at the same potential, ie the pd is zeroIs designed so that both input terminals are at the same potential, ie the pd is zero Can deliver a maximum output voltage which is equal to the value of its supply voltage.Can deliver a maximum output voltage which is equal to the value of its supply voltage. eg if a + 15V supply is used, max V out is 15V (but if a + 9V supply is used, max V out is only 9V) Trying to produce a higher output voltage than the supply will cause SATURATION to occur.Trying to produce a higher output voltage than the supply will cause SATURATION to occur.SATURATION

Voltage Gain For any amplifier, the voltage gain is defined as: V gain = V out V in eg, an amplifier with an input of 2V and an output of 10V, has a gain of 5 (no units are used for voltage gain)

An op-amp has a very high open-loop gain. This is controlled by using external resistors Controlling the Gain R 1 is an input resistor, R f is called a feedback resistor.

An op-amp can be used in several different ways, called the Modes (of operation). We will only look at 2 of them: Op-Amp Modes Inverting ModeInverting ModeInverting ModeInverting Mode Differential ModeDifferential ModeDifferential ModeDifferential Mode Other Modes can be found hereOther Modes can be found hereModes

Inverting Mode Only one voltage (V 1 ) is input to the op-amp It is applied to the inverting (-) terminal.Only one voltage (V 1 ) is input to the op-amp It is applied to the inverting (-) terminal. The non-inverting (+) terminal is connected to the ground (0V), making X also 0 Volts.The non-inverting (+) terminal is connected to the ground (0V), making X also 0 Volts. X

Inverting Mode Formula Negative sign shows that the output will be inverted compared to the input. This can be shown using ac voltage with a CRO. Input voltage Output voltage inverted phase

Example An inverting mode op-amp is set up with a 10k input resistor, and a 100k feedback resistor. An input voltage of 0.5V dc is applied. Calculate the output voltage.

Saturation Occurs when either the op-amp gain or the input voltage is too high.Occurs when either the op-amp gain or the input voltage is too high. If this occurs, the output voltage ‘saturates’ at the supply voltage value.If this occurs, the output voltage ‘saturates’ at the supply voltage value. eg if an op-amp is used with a + 15V supply and a gain of 10, the input is 2V ac, the output will be inverted but saturated with a value of 15V. 2V 15V

Differential Mode The purpose of this mode is to amplify the potential difference between 2 input voltages.The purpose of this mode is to amplify the potential difference between 2 input voltages. As before V 1 is applied to the inverting (-) terminal, now V 2 is applied to the non-inverting (+) terminal.As before V 1 is applied to the inverting (-) terminal, now V 2 is applied to the non-inverting (+) terminal. X R f R V1V1 V 0 0 V R2R2 R3R3 V2V2

Differential Mode Formula zero if V 2 =V 1 positive if V 2 >V 1 negative if V 2 <V 1 But ONLY if... The output will be:

Example This differential mode op-amp is set up. Calculate the output voltage.