Crystal oscillator with resistorless feedback biasing
View Patent ↗An oscillator includes an amplifier and a piezoelectric crystal coupled across a portion of the amplifier. A low pass filter (LPF) passes the common-mode voltage component of the crystal output signal. An auxiliary bias circuit uses a shared LPF component to charge a crystal load capacitor during start-up of the oscillator, and to provide a DC bias operating point to the oscillator driver transistor. A buffer amplifier receives the common-mode voltage component on the non-inverting input. The buffer amplifier output is coupled to both the inverting input and the drain terminal of the oscillator driver transistor such that the gate and drain DC bias voltages of the oscillator driver transistor are substantially the same. An automatic loop control circuit receives the crystal output signal and the common-mode voltage signal, and generates a bias control signal to bias the amplifier and the auxiliary bias circuit.
1. An oscillator that generates oscillator output signals at first and second oscillator output terminals of the oscillator, the oscillator comprising:
an amplifier coupled between a power rail and a low-voltage rail;
a piezoelectric crystal having first and second terminals, the crystal coupled across a portion of the amplifier with the first and second crystal terminals coupled to the first and second oscillator output terminals;
a load capacitance coupled between the first oscillator output terminal and the low-voltage rail;
a low pass filter (LPF) coupled to the first oscillator output terminal, wherein the LPF passes a common-mode voltage component to a LPF output terminal, and wherein the LPF has a shared LPF component;
an auxiliary bias circuit coupled to the first oscillator output terminal for charging the load capacitance using the shared LPF component during start-up of the oscillator and maintaining the amplifier biased to an operating point;
a buffer amplifier having a non-inverting input terminal, an inverting input terminal, and an output terminal, wherein:
the LPF output terminal is coupled to provide the common-mode voltage component to the non-inverting input terminal of the buffer amplifier; and
the output terminal of the buffer amplifier is coupled to the inverting input terminal of the buffer amplifier, and to the second oscillator output terminal; and
an automatic loop control (ALC) circuit coupled (i) to the first oscillator output terminal to receive an oscillator output signal and (ii) to the LPF output terminal to receive the common-mode voltage component, the ALC circuit generating a bias control signal based on the received oscillator output signal and the common-mode voltage component to bias the amplifier and the auxiliary bias circuit.
2. The oscillator of claim 1 , wherein the shared LPF component comprises a resistor.
3. The oscillator of claim 1 , wherein transistors in the auxiliary bias circuit have substantially smaller size than corresponding transistors in the amplifier.
4. The oscillator of claim 3 , wherein the size of the transistors in the auxiliary bias circuit range from approximately 1/100 th to approximately 1/200 th of the size of the corresponding transistors in the amplifier.
5. The oscillator of claim 1 , wherein the piezoelectric crystal comprises a quartz crystal.
6. The oscillator of claim 1 , further comprising:
a peak detector coupled to the first oscillator output terminal to detect a peak magnitude of the received oscillator output signal, wherein the ALC circuit develops the bias control signal based on the peak magnitude of the received oscillator output signal in addition to the common-mode voltage component.
7. The oscillator of claim 6 , wherein the peak magnitude of the received oscillator output signal is a negative magnitude.
8. An integrated circuit including the oscillator as recited in claim 1 .