colpitts osc not working

Description: The circuit presented is incorrect. Firstly, there is no DC path to the transistor base, which means there is no base bias current available. The collector and emitter of Q1 are directly connected to the battery without any impedance, preventing the transistor from delivering any useful output. Clarification is needed on how to determine the values for R1, R2, Re, RL, and Ce, as there is a lack of understanding regarding resistor biasing. C1 and C2 in the tutorial should both represent the load capacitance (18pF in this case), which is clear, but other aspects remain confusing. A reasonable collector current, which should exceed 1mA for the given values, is necessary to initiate circuit operation. In practical scenarios, noise voltages can trigger this, but simulations may require a manual input. A transient analysis using software such as PSpice could involve placing a pure current source in parallel with the crystal and applying a small pulse (1mA for half the crystal period, approximately 33ns for a 15MHz crystal). It is important to disconnect C1 from the Q1 base while ensuring Q1 remains biased. After verifying the bias, connecting a signal generator to the Q1 base and measuring the voltage at C1 (where the connection was broken) can indicate whether oscillation occurs if the voltage at C1 exceeds the signal generator voltage. If not, reducing C1 may be necessary. The second circuit functions correctly, while the first circuit is under investigation to understand the discrepancies between the 15MHz (working) and 25MHz (non-working) circuits. The capacitors in parallel with the crystal are 100pF each, and when combined, they present an abnormally high load capacitance to the crystal. Additionally, there is a need to calculate the voltage across the base and emitter, noting that the base must exceed the emitter voltage by at least 0.6V to activate the transistor.

To construct a functional oscillator circuit using a transistor, it is essential to establish a proper DC biasing network for the transistor base to ensure adequate base current is supplied. The absence of a DC path in the initial design results in an inoperable circuit, as the transistor requires a certain base-emitter voltage (approximately 0.6V) to enter the active region.

In the schematic, the resistors R1 and R2 form a voltage divider that sets the base voltage for Q1. The values of R1 and R2 can be calculated based on the desired base current and the supply voltage. The emitter resistor, Re, provides thermal stability and helps set the emitter current, while RL serves as the load for the collector. The load capacitance, represented by C1 and C2, should be carefully selected to match the specifications of the crystal oscillator, which operates at the desired frequency (15MHz or 25MHz).

The transient analysis in PSpice can be initiated by introducing a current pulse to stimulate the circuit, especially when simulating the initial conditions for oscillation. The interaction between the crystal and the capacitors should be analyzed to ensure the total load capacitance does not exceed the crystal's specifications, which could lead to frequency instability or failure to oscillate.

To measure the voltage across the base and emitter, one can place a multimeter or an oscilloscope probe between these two points. This measurement is crucial for verifying that the transistor is correctly biased and operating within the intended parameters. If the voltage does not meet the required threshold, adjustments to the biasing resistors or the load capacitance may be necessary to achieve stable oscillation.

In summary, a comprehensive understanding of the biasing network, load capacitance, and transient behavior is vital for successfully designing and troubleshooting oscillator circuits in electronic applications.I`m afraid the circuit you have there is nowhere near correct. For a start, there is no DC path to the transistor base, so no base bias current is available. Q1 collector and emitter are connected directly to the battery with no intervening impedance, so the transistor cannot deliver any useful output. Hmm. ok. so how do I figure out what R1, R2, Re, RL, and Ce should be I guess I don`t understand what resistor biasing does/means. C1 and C2 in that tutorial should both be the load capacitance (18pF in this case), so that makes sense, but nothing else really does *sadface* Once you have a reasonable collector current, which looks as if it should be a bit over 1mA, for the values you have, the next step is to give the circuit a nudge to try to start it. In the real world noise voltages should do this, but a simulation may need an actual kick. I suppose that you are doing a Transient analysis using something like PSpice. You could try putting a pure current source in parallel with the crystal, and applying a small pulse, say 1mA high for a pulse length of half the crystal period (33ns for a 15MHz crystal).

Break the connection between C1 and Q1 base. Make sure Q1 base is still biased. Verify Q1 bias as shown above. Then, connect a signal generator to Q1 base, and measure the voltage at C1, where you broke the connection. If the voltage at C1 is a little higher than the signal generator voltage, then you should get oscillation.

I am guessing it won`t be, and you`ll have to make C1 smaller. well the 2nd circuit I posted works fine, it`s the first one that I`m trying to create to understand what is going on. there is something I`m not understanding so let me break down the pieces that don`t make sense in the 15Mhz circuit (working) to try and make sense of the non-working 25Mhz circuit.

Those green arrows in parallel with the crystal, they are 100pF each, and when in parallel you add them together, so does that mean the crystal is receiving 100 load capacitance on each lead Abnormally high if that`s the case. How do I calc the voltage going across the base and emitter I understand there has to be a 0. 6V+ greater than the emitter on the base in order to turn the transistor on.