LC Oscillators

LC Oscillators

Lecture Outcomes

• Understand LC oscillators.
• Analyze LC oscillator circuits.
• Design Colpitts oscillators.
• Design Hartley oscillators.

Topics

• LC oscillators
• Analysis of LC oscillators
• Colpitts oscillators
• Hartley oscillators

LC Oscillators (3-element / Pi (π) type)

• The circuit includes an op-amp small signal model and a Pi feedback network (Z1, Z2, Z3).
• Feedback exists between the output voltage vo and the input voltage vi.
• v_p is the voltage at the non-inverting input.
• vn is the voltage at the inverting input which is equal to vi.
• A_0 is the open-loop gain of the op-amp.
• R_o is the output resistance of the op-amp.

Analysis

• Considering the node at vi = vn:
  \frac{vi - vo}{Z3} + \frac{vi}{Z1} = 0 vo = Z3 \left( \frac{vi}{Z3} + \frac{vi}{Z1} \right) = Z3 \left( \frac{1}{Z3} + \frac{1}{Z1} \right) v_i

• Considering the node at v0: \frac{vo - vi}{Z3} + \frac{vo}{Z2} + \frac{vo - v{\text{op-amp output}}}{Ro} = 0 \frac{v{\text{op-amp output}}}{Ro} = \frac{vo}{Z3} + \frac{vo}{Z2} + \frac{vo}{Ro} - \frac{vi}{Z3} = \left( \frac{1}{Z3} + \frac{1}{Z2} + \frac{1}{Ro} \right) vo - \frac{1}{Z3} v_i

• Substitution:

  v{\text{op-amp output}} = \left(1 + \frac{Z3}{Z1}\right) \left( \frac{Ro}{Z3} + \frac{Ro}{Z2} + \frac{Ro}{Ro} \right) vi - \frac{Ro}{Z3}v_i

Analysis (cont.)

• Deriving the transfer function:
  \frac{v{\text{op-amp output}}}{vi} = \frac{\frac{Ro}{Z3} + \frac{Ro}{Z2} + \frac{Ro}{Ro} + \frac{Z3}{Z1} \frac{Ro}{Z3} + \frac{Z3}{Z1} \frac{Ro}{Z2} + \frac{Z3}{Z1} \frac{Ro}{Ro} - \frac{Ro}{Z3}}{\frac{Z1}{Z1}} = \frac{\frac{Ro}{Z2} + \frac{Ro Z3}{Z1 Z2} + Ro \frac{Z3}{Z1}}{\frac{Z1}{Z1}} = \frac{\frac{Z1 Ro}{Z1 Z2} + \frac{Ro Z3}{Z1 Z2} + \frac{Ro Z2}{Z2} }{\frac{Z1}{Z1}} = \frac{Z1Ro + Z1Z2 + Z2Ro + Z3Ro + Z2Z3}{Z1Z2}
  = \frac{Ro (Z1 + Z2 + Z3) + Z2 (Z1 + Z3)}{Z1Z_2}
• Feedback factor:
  K(j\omega) = \frac{vi}{v{\text{op-amp output}}} = \frac{Z1Z2}{Ro (Z1 + Z2 + Z3) + Z2 (Z1 + Z_3)}
• Since Zn = jXn with Xn = XL = \omega L or Xn = XC = -\frac{1}{\omega C}:
  K(j\omega) = \frac{Z1Z2}{Ro (Z1 + Z2 + Z3) + Z2 (Z1 + Z3)} = \frac{-X1X2}{jRo (X1 + X2 + X3) - X2 (X1 + X3)}

Analysis (cont.)

• Barkhausen criteria application:
  K(j\omega) = \frac{-X1X2}{jRo (X1 + X2 + X3) - X2 (X1 + X_3)}

• Phase condition requires a real-valued equation:
  X1 + X2 + X_3 = 0

• Gain condition:
  K(\omega0) = \frac{-X1X2}{-X2(X1 + X3)} = \frac{X1}{X1 + X3} = -\frac{X1}{X_2}

• X1 and X2 are typically the same component type, their ratio is positive.

• If the gain of the amplifier with negative feedback is A, the total gain is:

  -A \cdot -\frac{X1}{X2} = |1| \angle 0^{\circ}

• A practical system requires a gain of -A to cancel the negative sign of K(\omega_0).

• The negative sign indicates the need for an inverting amplifier to correct the phase of K(\omega_0).

Colpitts Oscillator

• Circuit diagram includes capacitors C1, C2 and inductor L3.
• Derivation of design equations:
  Z1 = \frac{1}{j\omega C1}, Z2 = \frac{1}{j\omega C2}, Z3 = j\omega L3
• Satisfying the phase condition:
  X1 + X2 + X3 = 0 \implies -\frac{1}{\omega C1} - \frac{1}{\omega C2} + \omega L3 = 0
• Solving for \omega0: \omega L3 = \frac{1}{\omega} \left( \frac{1}{C1} + \frac{1}{C2} \right)
  \omega^2 = \frac{1}{L3} \cdot \left( \frac{1}{C1} + \frac{1}{C2} \right)^{-1} = \frac{1}{L3} \cdot \frac{C1 C2}{C1 + C2}
  \omega0 = \omega = \frac{1}{\sqrt{L3 \frac{C1 C2}{C1 + C2}}}
• Satisfying the gain condition:
  K(\omega0) = -\frac{X1}{X2} = -\frac{1/\omega C1}{1/ \omega C2} = -\frac{C2}{C_1}
• Total gain achieved with an inverting amplifier:
  -A \cdot -\frac{X1}{X2} = 1 \implies A = \frac{C1}{C2} = \frac{R2}{R1}

Hartley Oscillator

• Circuit diagram has inductors L1, L2 and capacitor C3.

• Derivation of design equations:

  Z1 = j\omega L1, Z2 = j\omega L2, Z3 = \frac{1}{j\omega C3}

• Satisfying the phase condition:

  X1 + X2 + X3 = 0 \implies \omega L1 + \omega L2 - \frac{1}{\omega C3} = 0

• Solving for \omega0: \omega0 = \omega = \frac{1}{\sqrt{C3 (L1 + L_2)}}

• Satisfying the gain condition:
  K(\omega0) = -\frac{X1}{X2} = -\frac{\omega L1}{\omega L2} = -\frac{L1}{L_2}

• Total gain achieved with an inverting amplifier:

  -A \cdot -\frac{X1}{X2} = 1 \implies A = \frac{L2}{L1} = \frac{R2}{R1}

LC Oscillator Example

• Design a Colpitts oscillator for f_0 = 10 \text{ kHz} and A = -10 \text{ V/V}.
• A circuit diagram with components R1, R2, C1, L3, C2 and the output voltage vout is shown.