RAMP GENERATOR
In electronics and electrical engineering, a ramp generator is a circuit that creates a linear rising or falling output with respect to time. The output variable is usually voltage, although current ramps can be created. Linear ramp generators are also known as sweep generator
Ramp generators produces a sawtooth wave form
Applications of ramp generators
Voltage and current linear ramp generator find wide application in instrumentation and communication systems. Ramp generators used in electrical generators or electric motors to avoid transients when changing a load
Linear current ramp generators are extensively used in television deflection systems.
There are many types of ramp generators, some of which are:
1. Constant current ramp generator
2. UJT relaxation generator
3. Bootstrap ramp generator
4. Miller integration ramp generator
This writeup will focus mainly on two of the above listed types i.e. The Bootstrap ramp generator and the Miller integration ramp generator.
BOOTSTRAP RAMP GENERATOR
Bootstrapping in electronics is a simple technique where part of the output of system is used at startup.
The process of bootstrapping is used to achieve constant charging. This will increase or decrease the input impedance of the circuit
The circuit diagram above is the schematic circuit diagram of a Bootstrap ramp generator. It consists of two transistors Q1 which acts as a switch and Q2 which acts as an emitter follower i.e. Unity gain amplifier.
Operation of Bootstrap ramp generator
Transistor Q2 turns off whenever negative pulse is applied to its base, The negative trigger pulse is from a gating waveform of a monostable multivibrator and is applied at the base of Q1 which turns it off. Capacitor C2 now discharges and the capacitor C1 charges through the resistor. Since the transistor Q2 acts as a unity gain amplifier, the output voltage is the same as the base voltage of transistor Q2. A s a result, transistor Q1 is turned off and capacitor C1 starts charging capacitor C2 through resistor R3
The base of transistor Q2 begins to increase in voltage and then consequently increase from zero. As the output increases the diode D1 becomes reverse biased. Voltage drop across capacitor C1 remains constant, since the value of capacitor C1 is much larger than capacitor C2, This cause the voltage across resistor R1 to remain constant, the current flowing through resistor also remains constant and then the voltage across the capacitor C1 increase linearly with time. Since C2 has high value, it discharges at a very slow rate.
The capacitor C2 which helps in providing some feedback current to the capacitor C1 acts as a boot strapping capacitor that provides constant current.
The image above shows the input negative pulse sent to transistor Q1 and also the saw-tooth output waveform.
ADVANTAGES OF BOOTSTRAP RAMP GENERATOR
The main advantage of this boot strap ramp generator is that the output voltage ramp is very linear and the ramp amplitude reaches the supply voltage level
Miller integrator generator
Also called a Miller integrator. In this setup, transistor Q1 acts as a switch and transistor Q2 is a common emitter amplifier i.e. A high gain amplifier.
Circuit diagram of a Miller integrator generator
To explain the operation of this circuit, we will assume that the transistor Q1 is ON and Q2 is OFF initially. This will cause the voltage across the capacitor to equal the output voltage Vcc. If we assume a negative pulse is applied at the base of the transistor Q1, its emitter base junction will be reverse-biased and it will turn off, the transistor Q2 will turn ON. The input is a pulse or a rectangular wave form.
The output voltage begins to decrease towards zero, this decrease is controlled by capacitor C1 which is coupled to the base of transistor Q2. The time constant of discharge is given by R2C. The collector voltage is linear as the value of time constant is very large, the discharge current remains constant.
When Q1 is on, the collector is at it’s low 0.3V for Silicon transistor and 0.7V for Germanium transistor, voltage which is not sufficient to on the transistor Q2 so the transistor Q2 remain off.
When Vinput is negative, Q1 is off and the voltage at the collector becomes maximum, hence its high enough to drive the transistor Q2, Once it is on, the capacitor starts to discharge. The result of this is that C2 discharges linearly, thus the voltage across capacitor is a negative going ramp.
Once the input negative pulse is removed, the transistor Q1 turns ON and transistor Q2 turns OFF. As this happens, the capacitor quickly charges through the resistor R3 to Vcc with time constant equal to RCC.
When Q1 is on, during the positive cycle, charging is through R3 and C2. Tr = Rc.C2, where Tr is the Return time.
Then when Q1 is off during the negative cycle, the discharging is through RC1 and Cs. Ts = RC1.C2, where Ts is the sweep time.
Output of the Miller integrator generator on virtual oscilloscope
The yellow saw-tooth waveform shows the output waveform of a Miller integrator, while the blue square wave shows the input wave to the base of transistor Q1.
Applications of Miller integrator
Miller sweep circuits are the most commonly used integrator circuit in many devices. It is a widely used saw tooth generator.
Here are some little electronic circuits showing
- Transistor as a switch
- 555 timer as astable multivibrator
Schematic diagram of transistor as a switch using Proteus
Physical diagram of transistor as a switch using fritzing
Schematic diagram of 555-timer as astable multivibrator, showing also it’s output signal
