Semiconductor Amplifiers

Constant current sources are circuits which provide a current to a load which is independent of the load resistance or the voltage across the load.

A transistor is a suitable device to produce such a constant current. As we have seen, the collector current of a transistor is almost independent of the collector emitter voltage. So, if we provide a constant voltage or current to the base, we will get a constant current at the collector.
The transistor has to be operated in the active region.

Fig. 5.1.1:
A transistor, used as constant current source. The elements R₁, R₂ and Z are used to provide the constant base current I_B. The output current I_o will then depend mainly on I_B, which can be controlled by R₂.

For higher output currents this circuit does not work satisfactory.
This is because the dynamic collector-emitter resistance decreases at higher collector currents.

The output current will also depend on the current gain of the transistor.

The circuit can be improved by using a current sense resistor. This resistor is connected in series with the emitter. The base of the transistor is connected to a constant voltage in respect to ground.

Fig. 5.1.2:
A transistor as constant current source with current sense resistor R_S. R₁ and Z provide a constant voltage

U_ref

to the base. The output current flows through R_S and provides a current dependent voltage.

The voltage across the current sense resistor must fulfil the following relationship:

U_RS = I_o*R_S = U_ref - U_BE

Therefore the output current Io will depend on the circuit constants

Either R_S or U_ref can be varied to set a required output current.

These circuits are frequently used to provide constant d.c. voltages to any equipment. They are also called voltage stabilizers or voltage regulators.

The output voltages of these circuits should neither vary with load current nor with input voltage. These qualities are described by the terms LOAD REGULATION and LINE REGULATION.

Fig. 5.2.1:
A voltage stabilizer is used to provide a constant output voltage U_o, which is independent of the load current I_L and the input voltage U_i.

LOAD REGULATION:

Describes how the output voltage varies with load current

This definition shows, that the load regulation is just the internal resistance of the constant voltage source.
The quality of the regulator is the better, the smaller the load regulation is.

Sometimes the load regulation is given in terms of a voltage variation for a specified current variation.

LINE REGULATION:

Also called VOLTAGE REGULATION. It describes how the output voltage varies with the input voltage:

Sometimes the line regulation is given in terms of output voltage variation for a specified input voltage variation.

Of cause voltage stabilizer circuits will only work for a limited range of input voltages and output currents.

Transistorized voltage stabilizers consist of the following parts:

• REFERENCE VOLTAGE SOURCE
  This is in almost all of the practical cases a zener diode with series resistor. For high quality requirements a reference element may be used.



• SERVO AMPLIFIER
  This is a d.c. amplifier. It compares the output voltage of the regulator with the reference voltage. Whenever there is a difference (error) the servo amplifier amplifies this difference and controls the servo regulator to compensate for the difference. The servo amplifier may be a transistor amplifier circuit, an operational amplifier or a special integrated regulator circuit.



• SERVO REGULATOR
  This part actually influences (regulates) the output voltage. It is controlled by the servo amplifier. Normally this is the collector emitter path of a transistor.

The simplest form of a transistorized voltage stabilizer has only one transistor. It serves as servo amplifier and regulator at the same time.

Fig. 5.3.1:
Simple transistorized voltage stabilizer. The zener diode provides the reference voltage. The transistor serves as servo amplifier and regulator.

The output voltage of this stabilizer will be:

U_o = U_z - U_BE

The output voltage will be constant, because U_z and U_BE can be assumed to be constant.

The function of the circuit can be explained in the following way:

If the load current increases, U_o would tend to drop. This would immediately increase U_BE, because U_BE = U_z - U_o. As U_BE increases, it will increase I_B and thus allow more collector current. In this way U_o is prevented from dropping. U_o remains constant. The transistor dissipates the voltage difference between U_i and U_o while carrying the load current. Sufficient cooling must be provided.

The circuit will also maintain U_o constant when U_i varies, as the voltage across the zener diode Uz is more or less independent of U_i.

This circuit will give satisfactory results for a few 100mA of output current. But it has the following disadvantages:

1. The current through the zener diode depends strongly on the load current.



2. For higher output currents the value of R becomes low and the current through the zener diode high.



3. The output voltage can not easily be varied.
   For different output voltages different zener diodes are required

For higher output currents and higher stability of U_o, one transistor is not sufficient. The "error" requires higher amplification. Two different measures to improve the circuit are required:

High current gain of the servo regulator
In order to make the zener current independent of the load current more current amplification of the servo regulator transistor is necessary. This may be achieved by using Darlington arrangements.

Fig. 5.3.2:
Improved voltage regulator circuit with Darlington servo regulator transistor.

High voltage gain of the servo amplifier
The circuit is equipped with an addition servo amplifier stage. This gives an much better regulation and allows to make the output voltage independent of the zener voltage. So always an optimum zener diode (5V to 7V) can be selected.

Fig. 5.3.3:
Improved circuit with additional servo amplifier transistor. The output voltage of this regulator can be adjusted with the resistors R₁ and R₂.

Electronic stabilizer circuits can also reduce ripple (hum) on the d.c. supply voltage. The ripple rejection can be improved by connecting capacitors in parallel to the reference element.

In general:
The better the constancy of the reference source and the higher the error amplification, the better the regulation of U_o.

Voltage regulators with very good performance can be built using operation amplifiers. The differential inputs of the opamp is perfectly suitable as reference input (non-inverting input) and sense input (inverting input). Current boost transistors may be used as servo regulators to achieve output current of several amps.

Fig. 5.3.4:
Voltage stabilizer circuits using opamps give perfect results.
a. Circuit without separate transistors can be used for up to a few 10mA.
b. With separate current boosters output currents of several amps are achievable.

Power supply units often require means of protection against excessive currents and short circuit. This can be done with electronic means.

Fig. 5.4.1:
Voltage regulator circuit with current limiting. It is provided by R₅ and T₄.

The resistor R₅ is the current sense which measures the output current. If the voltage drop across R₅ exceeds 0.5V, T₄ will become conductive, reducing the reference voltage and therefore the output voltage.

For some applications so-called fold back of the characteristic is required.

Fig. 5.4.2:
Characteristic of a power supply with current limiting and with fold back current limiting.

The fold back characteristic is achieved by adding some of the output voltage to the reference of the current sense.

Fig. 5.4.3:
Voltage regulator circuit with current limiting and fold back characteristic.

If negative supply voltages are required, the servo regulator should be situated in the negative line. Therefore PNP transistors have to be used.

Fig. 5.5.1:
Negative regulators for negative supply voltage with PNP transistors.

Another kind of voltage regulator is the parallel type or shunt regulator. Here the servo regulator is in parallel with the load. The principle is based on controlling the current through R_s to keep the output voltage constant.

This type of stabilizer is normally not used in electronic equipment.

Fig. 5.6.1:
Voltage stabilizer with parallel servo regulator.

Today voltage stabilizers are in most cases built using integrated circuits. There are basically three different types of regulators.

1. Fixed voltage regulators
   These circuits have three terminals only in standard transistor cases (TO-3, TO-220, TO-66, TO-92).
   They are available for output voltages between 3V and 30V.
   The regulators are short circuit protected and over-temperature protected.
   Different categories are available for current from 100mA to 1.5A
   
   

   example:	positive regulators: 78XX (XX= output voltage)
   	negative regulators: 79XX




2. Variable voltage regulators
   These are three terminal integrated circuits with adjustable output voltage in standard transistor cases (input, output, adjust). The output voltage can be adjusted with two resistors from 1.25V upward.
   The devices includes short circuit and over-temperature protection.
   
   

   example:

   positive regulators: LM117, LM217, LM317




3. Universal voltage regulators
   These are multi-purpose circuits which provide features for all types of power supply applications. They provide generally only little output current (<100mA), so booster transistors have to be used for larger output currents.
   
   

   example:

   LM723

The efficiency of series transistor type voltage regulators is relatively poor.

Example:

30V DC input regulated to 15V DC output. The power dissipated in the control transistor is equal to the power drawn by the load, efficiency = 50%.

Another system therefore employs a switching circuit. Basically the current to the reservoir capacitor is pulsed at relatively high frequency, e.g. 30kHz to 100kHz. The mark-to-space ratio of the pulses (or the pulse frequency of pulses with constant width) is adjusted to provide a certain output voltage. Only as much power as necessary is passed to the output and the switching circuit itself needs little power. Therefore the efficiency is very high.

Switching converts have a voltage current relationship between input and output like transformers: The product of current and voltage (the power) at the input and at the output remains almost constant.

The switched mode regulators basically consist of the following elements:

1. The switching device S:
   By varying its pulse width ratio it controls the amount of energy which flows from the source to the load. Typically the switch is a fast power transistor or a power FET.



2. The inductor L:
   It serves as energy storage device. It provides the output current during the cut-off periods of the switching device.



3. The fly-back diode D:
   It is required to close the current path for the inductor during the cut-off periods of the switching device.



4. The smoothing capacitor C:
   In conjunction with the coil it serves as energy reservoir during the switching periods of the coil.

Forward converters regulate like analog converters from a higher voltage to a lower voltage.

Function:

During the on-period of the switch, current flows through L and C, storing energy in both of them. During the off-periode of the switch the energy stored in the coil will be transferred through the diode D to the capacitor.

Fig. 5.8.1.1:
Principle of the forward converter.

Fast bipolar transistors or power field effect transistors are used for fast, efficient switching of high and very high power. Switching type stabilized power supplies are likely to be used also in broadcast studio equipment in the future.

Boost converters allow to achieve a higher output voltage than the input voltage. The input current will then be higher than the output current. So this circuit acts like a step up transformer.

Function:

During the on-period current i_Lon flows through the coil storing energy. During the off-period the current i_Loff continues to flow, transferring the energy to the capacitor. The increase in output voltage over the input voltage is achieved by adding the voltage u_L to the input voltage.

Fig. 5.8.2.1:
Principle of the boost converter. The output voltage is always larger than the input voltage.

With this converter it is possible to produce negative output voltage from positive input voltages.

Function:

When the switch S is closed, an increasing current i_Lon flows, energy is transferred to the coil L. When the switch S opens, the current in the coil flows to charge the capacitor negative, transferring the energy to the capacitor.

Fig. 5.8.3.1:
Principle of the flyback converter. It creates a negative output voltage from a positive input voltage.

An additional advantage can be achieved, if the switched mode regulator is moved to the primary side of the mains transformer. Due to the high switching frequency transformers with very small cores and very few windings can be used. This reduces their cost and their weight and increases the efficiency.

Fig. 5.8.4.1:
Principle of a primary switched mode transformer regulator.

This configuration produces some new problems:

1. The mains voltage has to be rectified and smoothed first.
   Due to the high voltage (ca. 300V) this is more difficult and more expensive than for low secondary voltages.



2. DC current flows through the windings of the primary and the secondary of the transformer. This would produces saturation of the core. Additional windings will be required on the transformer to compensate to avoid saturation.



3. The control signals for the switching device must be galvanicly decoupled, to provide full galvanic separation between primary and secondary.
   This is typically achieved by using opto-coupler devices.

Switched mode voltage stabilizers require a circuit which controls the pulse-width ratio of the switching device to maintain a constant output voltage. Such a circuit is relatively complex. The following block diagram shows the functional groups required.

Fig. 5.8.5.1:
Block diagram of the control circuit for a switched mode voltage stabilizer.

Integrated circuits are used which provide functions for all requirements of such regulators. The circuitry for such devices has to be done according to the data sheets.

Fig. 5.8.5.2:
Example of a simple forward regulator with NE5561 control circuit.

Semiconductor Amplifiers