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Oscillators Oscillators are electronic circuits that produce a constant oscillating signal that can be a sinusoid, a square wave or a triangular wave. Oscillators are classified as linear or harmonic oscillators if their output is a sinusoidal waveform. 1. Feedback a. RC Wien bridge Phase shift Twin-T Quadrature Robinson b. LC Armstrong/Meissner Hartley Colpitts Gouriet/Clapp Vackář Cross-coupled Meacham bridge Seiler c. Crystal Pierce Butler 2. Negative resistance Oscillators are classified as nonlinear or relaxation oscillators if their output is a square, a sawtooth or a triangular waveform. 1. Multivibrators a. Astable b. Monostable c. Bistable 2. Ring 3. Pearson-Anson or neon lamp 4. Delay line 5. Royer Some oscillators are simply classified as generators and their output is a square or a triangular waveform: 6. Square wave generator 7. Square/triangle waves generator 8. Triangle/square waves generator
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Notes:
Oscillators such as Hartley, Colpitts and Gouriet/Clapp can be configured to be Voltage-Controlled Oscillators (VCO). The frequency of the oscillators depends on input voltage.
The Vackář oscillator is described as a Variable-Frequency Oscillator (VFO).
Its frequency can be tuned with a variable capacitor. Its output is nearly constant over its frequency range of operation.
. Crystal oscillators were developed in the 1920s and 1930s and provided
better frequency stability than tuned oscillators because they are affected by temperature to a much lower degree (they are more stable).
The Tri-tet oscillator is described as an Electron-Coupled Oscillator (ECO).
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RC oscillators RC oscillators contain resistors and capacitors. They are typically used for low frequency (audio range or up to 20kHz). Wien bridge oscillator The circuit was first conceived by Prussian physicist Max Wien in 1891. Because of limitations during his time, the circuit was not constructed until American engineer William Hewlett revisited it in 1939 for his master’s degree thesis. Shortly after that, Hewlett-Packard was founded and one of the company’s first products was a sine-wave oscillator based on the Wien bridge circuit. The final product proved to be very successful and it became very popular because it was stable and inexpensive.
U1
AD741
+3
-2
V+7
V-4
OUT6
OS11
OS25
R3
57
C2
22n
C1
22n
0
0
0 0
V2
10Vdc
V1
10Vdc
0
V+V-
V+
V-
R2
1k
R4
115
R1
1k
V
Wien bridge oscillator
The Wien bridge oscillator uses positive feedback which is provided by a bandpass filter made up by two RC circuits, one in parallel (R1 and C1) and one in series (R2 and C2). R4 is a variable resistor and R3, at the time of Hewlett, was a light bulb.
Time
100.00ms 100.05ms 100.10ms 100.15ms 100.20ms 100.25ms 100.30ms 100.35ms 100.40ms 100.45ms 100.50msV(U1:OUT)
-5.0V
0V
5.0V
Transient response of the Wien bridge oscillator
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The ideal set of parameters is the following:
RRR 21 CCC 21 1
2
3
4 R
R
R3, the light bulb, has a selected resistance:
573R
The value for R4 can be easily calculated:
1142 34 RR
The gain is given by the following expression:
32157
11411
3
4
R
R
V
VA
i
o
The oscillating frequency is:
kHznFkRC
f 234.72212
1
2
1
The oscillating frequency from the simulation is 6.896kHz. Note: modern Wien bridge oscillators use other nonlinear elements such as diodes, thermistors, field effect transistors or photocells in place of light bulbs.
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Quadrature oscillator This circuit uses positive and negative feedback to generate two sinusoidal waves of similar properties, one of them being a sine and the other one being a cosine. C1 through C3 must be matched. All resistors should also be matched, except for R1 which must be slightly less than R2 and R3 to cause the circuit to oscillate. D1 through D4 avoid clipping at the outputs (breakdown voltage is 8.1V).
V1 10Vdc V2 10Vdc
0 0
V-V+
V-
V+
U1
uA741
+3
-2
V+7
V-
4
OUT6
OS11
OS25
V-U2
uA741
+3
-2
V+7
V-
4
OUT6
OS11
OS25
V+
R3
1k
R2
1k
C3
1u
C1
1uC2
1u
0
0
COSINE
SINE
Dbreak
D2
Dbreak
D4Dbreak
D1
Dbreak
D3
R1
900
0
V
V
Quadrature oscillator
Time
2.980s 2.982s 2.984s 2.986s 2.988s 2.990s 2.992s 2.994s 2.996s 2.998s 3.000sV(D2:2) V(D4:2)
-10V
-5V
0V
5V
10V
Transient response of the quadrature oscillator
The oscillating frequency is:
HzRC
f 1592
1
where R=R2=R3 and C=C1=C2=C3. The oscillating frequency from the simulation is 161Hz.
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LC oscillators
LC oscillators contain inductors and capacitors. They are typically used for high frequency (radio range or above 20kHz). Hartley oscillator This circuit was invented by American engineer Ralph Vinton Lyon Hartley in 1915. The oscillating frequency of this circuit depends on L1, L2 and C1. The Hartley oscillator is the dual circuit of the Colpitts oscillator which follows next.
Q1
Q2N2222
R1
100k
C3
100nF
V1
10Vdc
L1
142.5uH
L2
15.8uH
C2
100nF
R2
20k
0
R3
2.2kC1
4nF
0
0
C4
100nF
R4
10k
V
V
Hartley oscillator
Time
24.980ms 24.982ms 24.984ms 24.986ms 24.988ms 24.990ms 24.992ms 24.994ms 24.996ms 24.998ms 25.000msV(R4:2) V(C4:1)
-10V
0V
10V
Transient response of the Hartley oscillator
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If the inductors are not coupled, L is given by:
21 LLL If the inductors are coupled, L is given by:
2121 LLkLLL
where k is the coupling coefficient, a number between 0 and 1. The oscillating frequency for the circuit is given by:
kHznFHLC
f 009.20043.1582
1
2
1
1
The oscillating frequency from the simulation is 200kHz.
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Colpitts oscillator This circuit was invented by American engineer Edwin Henry Colpitts in 1918. The oscillating frequency of this circuit depends on C1, C2 and L1. The Colpitts oscillator is the dual circuit of the Hartley oscillator discussed above.
V
V
Q1
Q2N2222
R1
100k
C3
100nF
R2
20k R3
2.2k
C1
40nF
C2
400nF
V1
10Vdc
0
0
0
L1
17.41uHC4
100nF
R4
10k
Colpitts oscillator
Time
24.980ms 24.982ms 24.984ms 24.986ms 24.988ms 24.990ms 24.992ms 24.994ms 24.996ms 24.998ms 25.000msV(V1:+) V(R4:1)
0V
5V
10V
15V
Transient response of the Colpitts oscillator
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The oscillating frequency for the circuit is given by:
kHz
nFnF
nFnFH
CC
CCL
f 026.200
40040
4004041.172
1
2
1
21
211
The oscillating frequency from the simulation is 200kHz.
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Gouriet/Clapp oscillator This circuit was independently discovered and first published by American electrical engineer James Kilton Clapp in 1948. However, the circuit was invented by Geoffrey George Gouriet and it was used as early as 1938 at the BBC but this was not made public until after World War II due to the fact the circuit was kept secret. Essentially, the Gourier-Clapp oscillator is a Colpitts oscillator with an additional capacitor in series with the inductor.
R4
10k
C3
1uF
0
R5
100Meg
V
V
Q1
Q2N2222
R1
100k
C4
100nF
R2
20kR3
2.2k
C1
100nF
C2
1uF
V1
9Vdc
0
L1
10uH
C5
100nF
0
Gouriet-Clapp oscillator
Note: R5 is placed to force PSpice A/D to start.
Time
29.980ms 29.982ms 29.984ms 29.986ms 29.988ms 29.990ms 29.992ms 29.994ms 29.996ms 29.998ms 30.000msV(V1:+) V(L1:1)
0V
4V
8V
12V
Transient response of the Gouriet-Clapp oscillator
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The oscillating frequency for the circuit is given by:
kHzFFnFFCCCL
f 346.1741
1
1
1
100
1
10
1
2
11111
2
1
3211
The oscillating frequency from the simulation is 166.666kHz.
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Vackář oscillator This circuit was published in a paper by Czech engineer Jiří Vackář in 1949 but he attributed the invention of the oscillator that dates back to 1945 to a firm called Radioslava in Czechoslovakia.
L2
6.2uH
L1
100uH
C1100pF
C2600pF
C37.2nF
C41nF
V1
5Vdc
00
J1
J2N3819
V
V
Vackář oscillator
Time
4.9980ms 4.9982ms 4.9984ms 4.9986ms 4.9988ms 4.9990ms 4.9992ms 4.9994ms 4.9996ms 4.9998ms 5.0000msV(V1:+) V(C1:2)
-25V
0V
25V
50V
Transient response of the Vackář oscillator
The oscillating frequency from the simulation is 2MHz.
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Multivibrators Multivibrators are circuits designed to implement a two-state logic system and they can be of three types: astable, monostable and bistable. Astable multivibrators constantly oscillate between two states. Monostable multivibrators can be placed in a transient state by an external signal and return to the initial stable state after a specific time. Bistable multivibrators stay in either of two stable states and alternate between them depending on an external trigger. Astable multivibrator This circuit constantly oscillates between two states.
R11k
R268k
R382k
R41k
C1
100nF
C2
150nF
0
VCC
VCC
0
V1
5Vdc
Q1
Q2N2222
Q2
Q2N2222
++ --
VV
Astable multivibrator
Time
0s 10ms 20ms 30ms 40ms 50ms 60ms 70ms 80ms 90ms 100msV(C2:2)
0V
2.5V
5.0VV(C1:1)
0V
2.5V
5.0V
SEL>>
Transient response of the astable multivibrator
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The astable multivibrator frequency depends on the values of R2, C1, R3 and C2:
HznFknFkCRCRT
f 534.7515082100682ln
1
2ln
11
2312
The frequency from the simulation is 72.49Hz. As shown in the simulation, the circuit oscillates between two states. Note: if R2=R3 and C1=C2 the duty cycle will be exactly 50%.
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Monostable multivibrator This circuit can be placed in a transient state by an external signal and return to the initial stable state after a specific time.
R11k
R25k
R41k
C1
600nF
0
VCC
VCC
0
V1
5Vdc
Q1
Q2N2222
Q2
Q2N2222
+ R3
68k
V2
TD = 20ms
TF = 10nsPW = 30msPER = 100ms
V1 = 0.7V
TR = 10ns
V2 = 0V
0
-
V
VV
Monostable multivibrator
Time
0s 10ms 20ms 30ms 40ms 50ms 60ms 70ms 80ms 90ms 100msV(C1:1) V(R3:1)
0V
2.5V
5.0V
V(C1:2)
250mV
500mV
750mV
-100mVSEL>>
Transient response of the monostable multivibrator
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As shown in the simulation, Q2 is initially on and Q1 is off. External signal V2 brings the base of Q2 down to 0V which turns off Q2 and turns on Q1. When V2 goes back up to 0.7V the circuits goes back to its initial state. The time the monostable multivibrator stays in the transient state depends on the values of R2 and C1:
msnFkCRt 079.260052ln2ln 12
Time
49.0ms 49.5ms 50.0ms 50.5ms 51.0ms 51.5ms 52.0ms 52.5ms 53.0msV(C1:1) V(R3:1)
2.5V
5.0V
-1.0VSEL>>
V(C1:2)
0V
250mV
500mV
750mV
Transient response of the monostable multivibrator (detail)
As shown above, the circuit goes back to the initial state after about 2ms.
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Bistable multivibrator This circuit stays in either of two stable states and it alternates between them depending on an external trigger. This circuit is also referred to as flip-flop because it can store 1 bit of information.
R11k
R41k
0
VCC
VCC
0
V1
5Vdc
Q1
Q2N2222
Q2
Q2N2222
R3
10k
R2
10k
Reset
TD = 30ms
TF = 10nsPW = 30msPER = 50ms
V1 = 0.7V
TR = 10ns
V2 = 0V
0
Set
TD = 10ms
TF = 10nsPW = 20msPER = 50ms
V1 = 0.7V
TR = 10ns
V2 = 0V
0
VV
VV
Bistable multivibrator
Time
0s 10ms 20ms 30ms 40ms 50ms 60ms 70ms 80ms 90ms 100msV(R2:2) V(R3:1)
2.5V
5.0V
-1.0VSEL>>
V(Q2:b) V(Q1:b)
0V
250mV
500mV
750mV
Transient response of the bistable multivibrator
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As shown in the simulation, at 0ms, Q1 and Q2 are initially on so the circuit is in an undetermined state. At 10ms a 20ms external trigger called Set brings the base of Q2 down to 0V which turns off Q2 and turns on Q1. At 30ms a 30ms external trigger called Reset brings the base of Q1 down to 0V which turns off Q1 and turns on Q2. The circuit alternates between two states. It is in one state at 10ms-30ms and 60ms-80ms. It is the opposite state at 30ms-60ms and 80ms-100ms.
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Square wave generator This circuit uses positive and negative feedback in order to generate a square wave.
U1
uA741
+3
-2
V+7
V-4
OUT6
OS11
OS25
C1
1u
R1
3k
R2
3k
R31k
0
0
V1 2Vdc V2 2Vdc
0 0
V+ V-
V+
V-
V
Square wave generator
Time
40ms 42ms 44ms 46ms 48ms 50ms 52ms 54ms 56ms 58ms 60msV(R1:2)
-2.0V
0V
2.0V
Transient response of the square wave generator
The oscillating frequency is:
Hz
R
RCR
f 326
12
ln2
1
2
311
The oscillating frequency from the simulation is 343Hz.
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Square/triangle waves generator This circuit generates square and triangular waveforms. U2 integrates the output of U1 and flips it.
C1
1u
R2
1k
R1
1k
R3
1k
0
V1 2Vdc V2 2Vdc
0 0
V+ V-
V+
V-U1
uA741
+3
-2
V+7
V-4
OUT6
OS11
OS25
V-U2
uA741
+3
-2
V+7
V-4
OUT6
OS11
OS25
0V+
V
V
SQUARE
TRIANGLE
Square/triangle waves generator
Time
40ms 42ms 44ms 46ms 48ms 50ms 52ms 54ms 56ms 58ms 60msV(R3:1)
-2.0V
0V
2.0V
SEL>>
V(U1:OUT)-2.0V
0V
2.0V
Transient response of the square/triangle waves generator
The oscillating frequency is:
HzR
R
CRf 250
4
1
3
2
1
The oscillating frequency from the simulation is 255Hz.
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Triangle/square waves generator This circuit generates square and triangular waveforms. The frequency of the outputs can be modulated by the input voltage so this circuit is a Voltage-Controlled Oscillator (VCO).
U1
uA741
+3
-2
V+7
V-
4
OUT6
OS11
OS25
U2
uA741
+3
-2
V+7
V-
4
OUT6
OS11
OS25
C1
25nF
R1
50k
R3
51k
R4
51k
R2
25k
R5
10k
R6
100k
R7
51k
0
0
0
V1
5Vdc
0
V2
10Vdc
0
V3
10Vdc
0
V+ V-
V+
V+
V-
V-
SQUARE
TRIANGLE
Q1
Q2N2222
V
VV
Triangle/square waves generator
Time
0s 2ms 4ms 6ms 8ms 10ms 12ms 14ms 16ms 18ms 20ms 22ms 24ms 26ms 28ms 30msV(V1:+) V(C1:2) V(U2:OUT)
-10V
-5V
0V
5V
10V
Transient response of the triangle/square waves generator
For this circuit the ratio R1/R2 must be fixed to 1/2. Reducing the value of C1 by half doubles the frequency. Increasing the value of V1 by two also doubles the frequency which confirms this is a VCO. The oscillating frequency from the simulation is about 151Hz. The circuit can be implemented with a MOSFET instead of a BJT.
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555 timer IC The 555 timer IC was designed by Swiss electronics engineer Hans R. Camenzind in 1970 or 1971 and introduced on the market by Signetics in 1971. It is a classical circuit and it can be configured to function just like a multivibrator, an oscillator or a flip-flop with the addition of a few external components such as capacitors and resistors. Internally, the IC can be implemented with BJTs or MOSFETs. Depending on the manufacturer and the implementation, the 555 timer IC has transistors, resistors and diodes. The chip is available in an 8-pin configuration. The 556 version of the timer has 14 pins and it contains 2 555 chips. 558 and 599 versions come in 16-pin chips and they contain 4 modified 555 chips. The NE555 was the first 555 timer IC chip, it was released by Signetics and it was designed for operation between 0°C and +70°C. The SE555 was designed for the military with a temperature range of -55°C to +125°C. A V suffix was used for a plastic package and a T suffix was used for the metal package so that, for example, the SE555T was a military metal packaged 555 timer IC. The 555 timer IC has the following pins:
The chip can be configured to work in the following modes:
Astable or free-running mode Monostable or one-shot mode Bistable or Schmitt-trigger mode
For the following simulations, the TLC555 by Texas Instruments is configured to work in the astable, monostable and bistable modes. Pin 5, the control pin, is always connected to a 10nF capacitor that goes to ground.
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The internal schematic of the BJT implementation of the LM555 timer IC by Texas Instruments
The internal schematic of the CMOS implementation of the TLC555 timer IC by Texas Instruments
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Astable mode
The astable or free-running mode produces an astable multivibrator. For this circuit, the only external components needed are 2 resistors and 2 capacitors.
C1
15n
R1
3.3k
R2
1k
C2
10n
0
0
V1
5Vdc
0
0
Cont
Reset
Vdd
Disch Out
Thresh
Trig
GndU1TLC555
74
1
5
3
8
62
V
The 555 timer IC in astable mode
Time
29.50ms 29.55ms 29.60ms 29.65ms 29.70ms 29.75ms 29.80ms 29.85ms 29.90ms 29.95ms 30.00msV(U1:OUT)
0V
2.0V
4.0V
6.0V
Transient response for the 555 timer IC in astable mode
The oscillating frequency is:
kHzkknFRRC
f 174.18123.3152ln
1
22ln
1
211
The on-time for the pulse is:
skknFRRCton 71.4413.3152ln2ln 211
The off-time for the pulse is:
sknFRCtoff 40.101152ln2ln 21
The oscillating frequency from the simulation is 17.543kHz, the on-time is 46µs and the off-time is 11µs.
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Monostable mode The monostable or one-shot mode produces a monostable multivibrator. For this circuit the only external components needed are 1 resistor and 2 capacitors. Pin 2, the trigger pin, is active low. When the pin is brought low, it will initiate the pulse.
C1
15n
Cont
Reset
Vdd
Disch Out
Thresh
Trig
GndU1TLC555
74
1
5
3
8
62
R1
3.3k
0
C2
10n
0
V1
5Vdc
0
0
V2
TD = 5ms
TF = 1nsPW = 10usPER = 100m
V1 = 5V
TR = 1ns
V2 = 0V
0
V V
V
The 555 timer IC in monostable mode
Time
4.90ms 4.92ms 4.94ms 4.96ms 4.98ms 5.00ms 5.02ms 5.04ms 5.06ms 5.08ms 5.10msV(U1:OUT) V(C1:2)
0V
2.5V
5.0V
SEL>>
V(V2:+)0V
2.5V
5.0V
Transient response for the 555 timer IC in monostable mode The time of the pulse is:
snFkCRt p 38.54153.33ln3ln 11
The pulse ends when the voltage across C1 is 2/3 of the supply voltage.
The time of the pulse from the simulation is 55.1µs.
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Bistable mode The bistable or Schmitt-trigger mode produces a bistable multivibrator. For this circuit the only external component needed is 1 capacitor. Pin 2, the trigger pin, is active low. When the pin is brought low, it will set the output to a high state. Pin 4, the reset pin, is also active low. When the pin is brought low, it will reset the output to a low state.
0
C1
10n
0
V1
5Vdc
0
0
V3
TD = 7ms
TF = 1nsPW = 100usPER = 100m
V1 = 5V
TR = 1ns
V2 = 0V
0
V2
TD = 4ms
TF = 1nsPW = 100usPER = 100m
V1 = 5V
TR = 1ns
V2 = 0V
0
RESET
SET
Cont
Reset
Vdd
Disch Out
ThreshTrig
GndU1TLC555
64
1
5
3
8
72
V
V
V
The 555 timer IC in bistable mode
Time
3.0ms 3.5ms 4.0ms 4.5ms 5.0ms 5.5ms 6.0ms 6.5ms 7.0ms 7.5ms 8.0msV(U1:OUT)
0V
2.5V
5.0V
V(V3:+)
0V
2.5V
5.0V
V(U1:TRIG)
2.5V
5.0V
SEL>>
Transient response for the 555 timer IC in bistable mode