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Operational Amplifiers

Operational Amplifiers. Dr. Holbert February 11, 2008. Op Amps. Op Amp is short for operational amplifier Amplifiers provide gains in voltage or current Op amps can convert current to voltage. Applications of Op Amps.

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Operational Amplifiers

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  1. Operational Amplifiers Dr. Holbert February 11, 2008 EEE 202

  2. Op Amps • Op Amp is short for operational amplifier • Amplifiers provide gains in voltage or current • Op amps can convert current to voltage EEE 202

  3. Applications of Op Amps • Op amps can be configured in many different ways using resistors and other components • Most configurations use feedback • Op amps can provide a buffer between two circuits • Op amps can be used to implement integrators and differentiators • Lowpass and bandpass filters EEE 202

  4. + – The Op Amp Symbol High Supply Non-inverting input Output Inverting input Ground Low Supply EEE 202

  5. v+ Non-inverting input + vo Rin + – Inverting input – A(v+ – v–) v– The Op Amp Model • An operational amplifier is modeled as a voltage-controlled voltage source. EEE 202

  6. Typical Op Amp: The input resistance (impedance) Rin is very large (practically infinite). The voltage gain A is very large (practically infinite). Ideal Op Amp: The input resistance is infinite. The gain is infinite. The op amp is in a negative feedback configuration. Typical vs. Ideal Op Amps EEE 202

  7. Consequences of the Ideal • Infinite input resistance means the current into the inverting (–) input is zero: i– = 0 • Infinite gain means the difference between v+ and v– is zero: v+–v– = 0 EEE 202

  8. The Basic Inverting Amplifier R2 R1 – + – + + Vin Vout – EEE 202

  9. R2 Vout i2 R1 Vin V– – i1 i– Solving the Amplifier Circuit Apply KCL at the inverting (–) input: i1 + i2 + i–=0 EEE 202

  10. From KCL Thus, the amplifier gain is Solve for Vout EEE 202

  11. Recap • The ideal op-amp model leads to the following conditions: i– = 0 = i+ v+ = v– • These conditions are used, along with KCL and other analysis techniques (e.g., nodal), to solve for the output voltage in terms of the input(s) EEE 202

  12. Where is the Feedback? R2 R1 – + – + + Vin Vout – EEE 202

  13. Review • To solve an op-amp circuit, we usually apply KCL at one or both of the inputs • We then invoke the consequences of the ideal model • The op amp will provide whatever output voltage is necessary to make both input voltages equal • We solve for the op-amp output voltage EEE 202

  14. The Non-Inverting Amplifier + + – + – vin vout R2 R1 – EEE 202

  15. + + – + – i– vin vout i1 i2 R2 R1 – KCL at the Inverting Input EEE 202

  16. Solve for vout • Hence, the non-inverting amplifier has a gained output (> unity) relative to the resistance ratio EEE 202

  17. A Mixer Circuit R1 Rf + – R2 v1 – + – + + v2 vout – EEE 202

  18. R1 Rf i1 if + – R2 v1 i2 – i– + – + + v2 vout – KCL at the Inverting Input EEE 202

  19. Solve for vout • So, the mixer circuit output is a (negative) combination of the input voltages EEE 202

  20. Class Examples • Drill Problem P4-1 EEE 202

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