Topic 6 - Op Amps Flashcards
Chapters 5 & last part of 6 (24 cards)
What is an operational amplifier?
They are active elements (can supply voltage/current to a circuit). They are designed to perform mathematical operations in analog circuits when resistors and capacitors are connected to its terminals.
Pins of interest on an Op Amp?
Inverting input (-) (2)
Non-inverting Input (+) (3)
Output (6)
Positive power supply V+ (7)
Negative power supply (4)
Ideal model of Op Amp?
V1: Inverting Terminal voltage
V2: Non-inverting terminal voltage
Vd: (V2-V1) Differential Input voltage
Ri: Thevenin resistance at input terminals
A: Open-Loop voltage gain
Ro: Thevenin resistance at output terminals
Ideal values for Op Amp parameters? And typical range for supply voltage Vcc.
Ri: Infinite
A: Infinite
Ro: 0
Vcc: 5 - 24V
What is the significance of the input resistance?
Since the input resistance is so high, the current entering the input terminals is effectively 0A.
What is the output voltage of an Op Amp limited by?
It is capped at Vcc.
What mode of Op Amp operation are we interested in?
The magnitude of output voltage (Vo) cannot exceed its supply voltage.
It then operates in three modes, depending on Vd (differential input voltage):
- Positive saturation
- Negative saturation
- Linear region
We are interested in the linear region, when output voltage is in between Vcc values.
Formula for output voltage (Vo)?
Where A is open-loop voltage gain and Vd is differential input voltage.
What are the two important properties that are needed to analyse ideal Op Amps?
- The currents into both terminals is zero.
- The voltage difference across the input terminals is zero. (Only happens when there is negative feedback)
What is negative feedback?
When the output is connected to the inverting terminal. As V1 increases, Vd will decrease. This result in the output stabilising the input and gain, and will keep the Op Amp in the linear region.
What happens when you connect the output to the positive terminal?
When connecting output to the non-inverting terminal, Vd (differential voltage) will start to increase, as V2 increases. This increases gain (A) and increases output to saturation.
How do you solve Op Amp circuits?
- Assume Ideal Op Amp
- Apply nodal analysis at the terminals
- Solve nodal equations.
What is the formula for closed loop gain?
Output voltage (Vo)/Input voltage (Vs)
Formula for output voltage (Vo) in an inverting amplifier?
Reverses polarity of the input signal while amplifying it.
Formula for output voltage (Vo) in a non-inverting amplifier?
Does not reverse polarity of input signal during amplification.
What are voltage followers used for?
They have a very high input resistance, and is useful in isolating one circuit from another.
The second circuit does not draw current from the first circuit. Instead, the Op Amp provides current and power while maintaining the same voltage as the first circuit.
This removes any effect that the changing second circuit (the load) would have had on the first circuit.
What is a summing amplifier?
Special type of INVERTING Op Amp circuit that combines several inputs and produces a weighted sum for its output.
What is a voltage follower?
A special type of NONINVERTING Op Amp occurs when Rf = 0, or R1 = infinity, or both, so the gain becomes 1.
What is a difference amplifier?
Amplifies the difference between inverting and non-inverting inputs.
When is Va = Vb?
When there is a negative feedback.
What is the formula for a difference amplifier? Why?
We must take R1/R2=R3/R4.
What is cascade of Op Amps? What is the gain of a series of amplifiers in cascade?
A cascade is a chain of multiple Op Amp circuits (stages). Due to input and output impedances of IDEAL Op Amps, stages can be connected without any loading effect.
The gain of the series is the product of individual gains.
What is an integrator circuit?
When a capacitor replaces the feedback resistor in Op Amps, it integrates the input signal times (-1/RC) to give the output signal.
What is a differentiator circuit?
When a capacitor replaces the input resistor, the circuit will differentiate the input signal and multiply by (-RC) to give the output signal.
