Introduction
Within the Experimenting with magnetic components challenge I am conducting experiments to develop an smart coin discriminator using inductive sensitivity.
The coin discriminator needs an oscillator to generate the variable magnetic field with the LC tank.
I'm going to experiment with oscillator circuits with different inductors from the Bourns kit. Although they are inductors designed for other functions, I am going to see what I can learn from them about oscillators and specifically Colpitt oscillators.
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EXPERIMENTS
- #E05 Colpitts Oscillator - Ltspice simulations
- #E06 Colpitts Oscillator with Bourns RL622 3.3uH RF choke
- #E07 Colpitts Oscillator with Bourns 9250A 47uH RF choke
- #E08 Colpitts Oscillator with Bourns 1110 10uH High Current Choke
- #E09 Colpitts Oscillator with Isolated Output. Using Bourns HCT Pulse Transformer
- Conclusions
- References
Oscillators
An electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine wave or a square wave or a triangle wave. Oscillators convert direct current (DC) from a power supply to an alternating current (AC) signal.
Feedback oscillator
The most common form of linear oscillator is an electronic amplifier such as a transistor or operational amplifier connected in a feedback loop with its output fed back into its input through a frequency selective electronic filter to provide positive feedback.
Barkhausen stability criterion
The Barkhausen stability criterion is a mathematical condition to determine when a linear electronic circuit will oscillate.
It states that if A is the gain of the amplifying element in the circuit and β(jω) is the transfer function of the feedback path, so βA is the loop gain around the feedback loop of the circuit, the circuit will sustain steady-state oscillations only at frequencies for which:
- The loop gain is equal to unity in absolute magnitude, that is, |βA| = 1 and
- The phase shift around the loop is zero or an integer multiple of 2π:
Barkhausen's criterion is a necessary condition for oscillation but not a sufficient condition: some circuits satisfy the criterion but do not oscillate.
Colpitt oscillators
Colpitt oscillators are sine wave oscillators. A sine wave oscillator consists of an amplifier, a positive feedback loop, and a tuned circuit that ensures that the oscillation occurs at a single defined frequency. Also, there must be some method to stabilize the amplitude of the oscillations so that the oscillation does not stop or build up to an amplitude such that the wave is distorted due to the background or cut-off. A pure sine wave has only a single or fundamental frequency—ideally no harmonics are present.
The Colpitts circuit, like other LC oscillators, consists of a gain device, such as a bipolar junction transistor, field-effect transistor or operational amplifier, with its output connected to its input in a feedback loop containing a parallel LC circuit (tuned circuit), which functions as a bandpass filter to set the frequency of oscillation. The amplifier will have differing input and output impedances, and these need to be coupled into the LC circuit without overly damping it.
The Colpitts Oscillator is a good circuit for producing fairly low distortion sine wave signals in the RF range, 30kHz to 30MHz.
The Colpitts configuration can be recognized by its use of a tapped capacitor divider (C1 and C2 in the figure) to provide the feedback path.
The frequency of oscillation can be calculated in the same way as any parallel resonant circuit, using:
The actual frequency of oscillation will be slightly lower due to junction capacitances and resistive loading of the transistor.
The values of the two capacitors (connected in series) are chosen so their total capacitance in series(CTOT), is given by:
The individual values of C1 and C2 are chosen so that the ratio of the values produces the necessary proportion of feedback signal.
However, the ratio of voltages across two capacitors in series is in inverse proportion to the ratio of the values, i.e. the smaller capacitor has the larger signal voltage across it.
To begin with I am going to do experiments using a BJT NPN transistor for the amplifier. The amplifier can be made with different configurations such as common-base, common-emitter or common-collector.
In the first experiments I will use the common'base configuration.
Common-Base, also known as grounded-base, amplifier is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a current buffer or voltage amplifier. In this circuit the emitter terminal of the transistor serves as the input, the collector as the output, and the base is connected to ground, or "common", hence its name.
EXPERIMENTS
#E05 Colpitts Oscillator - Ltspice simulations
Build a simulation schematic of the Colpitts oscillator as shown below. Then calculate values for bias resistors R1 and R2 such that with emitter resistor R3 set to 1 KΩ, the collector current in NPN transistor Q1 is approximately 1 mA. The circuit is powered from a +10V power supply. To keep the standing current in the resistor divider as low as practical keep the sum of R1 and R2 ( total resistance greater than 10 KΩ) as high as practical . C3 provides an AC ground at the base of Q1. Set base decoupling capacitor C3 and output AC coupling capacitor C4 to 0.1uF.
The calculate a values for C1 and C2 such that the resonance frequency, with L1 set equal to 3.3 uH will be close to 3.049 MHz
C1 = 4.7nF, C2 = 1 nF, R1 = 10KΩ, R2 = 1KΩ
Simulation with LTSpice
Initially construct the oscillator with separate positive and negative DC voltage supplies .
Schematic
Transient simulation
FTT
Resonant frequency 2.620 MHz
#E06 Colpitts Oscillator with Bourns RL622 3.3uH RF choke
In this experiment I use a 3.3 RF Choke in a Colpitts oscillator circuit with an amplifier with BJT in common-base configuration.
Required components:
1 - 2N3904 NPN transistor
1 - 3.3 uH inductor
1 - 1 nF capacitor ( marked 102 )
1 - 4.7 nF capacitor ( marked 472 )
2 - 0.1 uF capacitors ( marked 104 )
1 - 5 KΩ potentiometer to find the best value for R3,
Product LinkProduct Link
RF Choke, Radial Lead, 3.3 uH
http://www.farnell.com/datasheets/2283900.pdf
I build the circuit on a breadboard. It would be better to have the components soldered but it is worth to see the behavior of the oscillators despite the noise.
For R3 I use a 5K potentiometer to adjust the value of the emitter resistor. It is a very critical value so that the wave is not clipping or disappearing.
The potentiometer allows to vary IE to tune the output voltage amplitude.
A plot example using R1=10KΩ, R2=1KΩ, C1=4.7nF, C2=1nF, L1=3.3uH
- C1 = 4.7nF,C2 = 1 nF,C3 = n/a,R1 = 10KΩ,R2 = 1KΩ
Frequency: 2.441 MHz
- C1 = 4.7nF, C2 = 1 nF, C3 = 47 nF, R1 = 15 K, R2 = 5K6
- C1 = 33 nF, C2 = 10 nF, C3 = 47 nF,R1 = 10 KΩ, R2 = 1K
Adjusting R3, emitter resistor
The optimal value for R3 may change depending on the resonant frequency.
When the emitter resistance is not well chosen, clipping and deformation of the signal occur.
#E07 Colpitts Oscillator with Bourns 9250A 47uH RF choke
In this experiment I use a 47uH RF Choke in a Colpitt oscillator circuit with an amplifier with BJT in common-base configuration.
Product LinkProduct Link
https://www.farnell.com/datasheets/2360326.pdf
Required components:
1 - 2N3904 NPN transistor
1 - 47 uH inductor
1 - 1 nF capacitor ( marked 102 )
1 - 4.7 nF capacitor ( marked 472 )
2 - 0.1 uF capacitors ( marked 104 )
1 - 5 KΩ potentiometer to find the best value for R3,
- L1 = 47 uH C1 = 33 nF C2 = 10 nF C3 = 47 nF R1 = 10 KΩ R2 = 1K
I build the circuit on a breadboard. he circuit is the same as the previous one by changing the inductor and adding a bypass capacitor in the feedback signal.
Wave Frequency: 886.5 kHz
#E08 Colpitts Oscillator with Bourns 1110 10uH High Current Choke
In this experiment I use a 10uH High Current Choke in a Colpitts oscillator circuit with an amplifier with BJT in common-base configuration.
1 - 2N3904 NPN transistor
1 - 10 uH inductor
1 - 1 nF capacitor ( marked 102 )
1 - 4.7 nF capacitor ( marked 472 )
2 - 0.1 uF capacitors ( marked 104 )
1 - 5 KΩ potentiometer to find the best value for R3,
10uH High Current Choke
Product LinkProduct Link
http://www.farnell.com/datasheets/167630.pdf
Wave Frequency 1.587 MHz
with c3
Observations
- As the inductance increases, the output voltage increases.
- Increasing the emitter current increases the amplitude of the output voltage.
- Decreasing the capacitance increases the output voltage.
- Decreasing the inductance decreases the resistance of the output.
- Controlling the emitter current is critical for the circuit to oscillate.
#E09 Colpitts Oscillator with Isolated Output. Using Bourns HCT Pulse Transformer
To finish the experiments with the Colpitts oscillators with BJT I am going to add a transformer to the output to isolate the output circuit from the oscillator circuit.
An isolation transformer is a transformer used to transfer electrical power from a source of alternating current power to some equipment or device while isolating the powered device from the power source, usually for safety reasons. Isolation transformers provide galvanic isolation and are used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected.
The kit has no isolation transformer but I use a pulse transformer that is in a kit with a 1: 2 ratio but with 1: 1 ratio as an isolation transformer.
Product LinkProduct Link
http://www.farnell.com/datasheets/2897691.pdf
Making and adapter for breadboard
before cleaning flux:
Configuration with 1:1 ratio
- Collector out to Primary 1-3
- Isolated output to Secondary 6-5
Measuring collector current. Near 22.8 mA
Probe 1 to transformer primary
Probe 2 to transformer secondary
The transformer works very well
Conclusions
I have learned the conditions necessary to generate an oscillation. It is possible to generate sinusoidal oscillators using inductors and capacitors and an NPN transistor as an amplifier. How the inductance and capacitance of the LC tank affect the frequency of oscillation and the voltage of the output signal. And finally I have used a transformer to isolate the oscillator from the circuit that will produce the variable magnetic field from the inductive sensor to the coin discriminator.
References
