The Basic Opamp

The Basic Opamp

The opamp was originally designed to carry out mathematical operations in analogue computers, such as bombsights, but was soon recognised as having many other applications. The opamp usually comes in the form of an 8 pin integrated circuit, the most common one being the type 741. It has two inputs and one output. The input marked with a  - sign produces an amplified inverted output. The input marked with a + sign produces an amplified but non inverted output. The opamp requires positive and negative power supplies, together with a common ground. Some circuits can be designed to work from a single supply. If the two inputs are joined together, then the output voltage should be midway between the two supply rails, i.e. zero volts. If it is not, then there are two connections for adding a potentiometer, to remove this OFFSET.  Setting Opamp Gain  The gain of the inverting amplifier is determined by the feedback resistor R2, and the input resistor R1. To minimize temperature drift, R3 is given the value of R1 and R2 in parallel. Non Inverter with Gain  Gain is 1+ R2/R1 Opamp Characteristics  The opamp has a very high gain, typically (100 dB)100,000 times. Looking at the left hand diagram, an input with a swing of a fraction of a millivolt produces an output that changes between + 12 volts and - 12 volts. In most cases this gain is excessive, and is reduced by negative feed back. Looking at the right hand diagram we can see that the opamp amplifies right down to dc. Gain falls quite rapidly as the frequency increases. In fact the bandwidth (the point at which the output has fallen by 3 dB) is only 1 kHz. This is also improved upon by the use of negative feedback. The input impedance is high, 1M. The output impedance is low, 150 ohms.

Metal Oxide Semiconductor Fet (Mosfet)

Metal Oxide Semiconductor Fet (Mosfet)

The mosfet has the gate insulated from the substrate by a thin layer of silicon oxide, to prevent gate current flowing and damaging the device (see the page on fets). There are two main families. Enhancement - where the mosfet has to be forward biased like a transistor. Depletion - where the mosfet is reverse biased like a thermionic valve (tube in the USA). Some mosfets have two gates (dual gate mosfets) and are commonly used as r.f. mixers. The insulating layer is extremely thin and can be easily damaged by static. Antistatic precautions must be taken when handling them. Soldering iron tips must be earthed. The operator must be grounded via a high value resistor, with wrist straps etc. The workplace must be grounded safely. Components must be handled with care. The operator should touch some earthed point just before handling static sensitive devices. Some devices have Zener diodes built in, between gate and source, for protection.  The thick line represents the channel and if it is unbroken represents a depletion ( normally conducting) type. If the channel is shown broken it is a normally enhancement (non conducting) type.

CE And CC Amplifier

Common Emitter Amplifier

Sometimes called the grounded emitter, since the emitter capacitor connects the emitter to ground at ac frequencies. Since, as far as ac is concerned, the emitter is joined to ground, both input and output are connected to the emitter. Current gain is Ic/Ib and can be quite high, typically 50. Voltage gain is high, typically 250 Input impedance is medium, say 5K. Output impedance is medium, say 20k. The output is inverted with respect to the input. Its most common application is as a voltage amplifier. Common Collector Amplifier  The positive power supply rail is joined to the zero volts rail by C3. As far as ac is concerned, both rails are joined together. Therefore they, and the collector,  are common to both input and output. Since the emitter voltage follows the base voltage, it is also called the emitter follower. Current gain is Ie/Ib which is quite high, typically 50. Voltage gain is only 1 because of the undecoupled emitter. The input impedance is high, typically 500k, requiring only low power to drive it. The output impedance is low, typically 20 ohms. The output signal follows the input. There is no inversion. It is often used to match high impedances to low ones. It can be used to drive several high impedance loads. Common Base Amplifier  C3 connects the base to ground as far as ac is concerned. Therefore both input and output are connected to the base. (common base amplifier). Current gain is Ic/Ie which is less than 1. The voltage gain is high since it is Rc/Re. (Approximately the same current flows through them).  It is typically 250. The input impedance is low, typically 20 ohms. The output impedance is high, typically 1Megohm. The output signal is not inverted with respect the input. It is often used to match low impedance devices to high impedance ones. It is commonly used at VHF.

Junction Transistors

Junction Transistors

Junction transistors consist of two junctions made from N-type and P-type semiconductor materials and are called bipolar transistors (two polarities). They have three connections, emitter, base and collector. Transistor Operation  The forward biased base/emitter junction causes electrons to be attracted from the emitter area towards the base. Arriving in the base area, most of the negative electrons come under the influence of the more positive collector and are attracted by it. This is shown in the left hand drawing, where the base current plus collector current equals the emitter current. Alpha gain is collector current divided by emitter current, and is always less than 1. Beta gain is collector current divided by base current and can be a fairly high number. Therefore, causing a small base current to flow makes a much larger collector current flow. A small base current controls a large collector current. There is 0.6 volts across the base/emitter junction, when it is forward biased. (0.3 volts for germanium). Biasing a Transistor  Choose a general purpose transistor with a beta gain higher than 100. Decide on the collector current. The base bias voltage is be 1/3 of the supply voltage. The current through the base bias potential divider is to be 1/10 of the collector current. Calculate the two base resistor values, R1 and R2. The emitter voltage is 0.6 volts lower than the base voltage. The value of the emitter resistor R4 is the emitter voltage divided by the collector current. The value of the collector resistor R3 is the supply voltage divided by three times the collector current. The values of the capacitors depend upon the application. Study a few circuits. For common collector and common base some of the capacitors are connected differently.

Introduction to Diodes

Diodes are polarised, which means that they must be inserted into the PCB the correct way round. This is because an electric current will only flow through them in one direction (like air will only flow one way through a tyre valve). Diodes have two connections, an anode and a cathode. The cathode is always identified by a dot, ring or some other mark. The pcb is often marked with a + sign for the cathode end. Barrier  At the junction, electrons fill holes so that there are no free holes or electrons there. The actual junction becomes an insulating layer. This barrier must be overcome before current can flow through the P-N junction. Diode Characteristic Curves  An electronic gate opens to let part of a signal through, and then shuts to reject the remainder. It's like separating sheep from goats, using a real gate. In the circuit, the cathodes of the diodes are more positive than the anodes. They are reverse biased and non conducting. The output of the circuit is isolated from the input. When the negative gating pulse comes along, the cathodes become more negative than the anodes. The diodes are forward biased and conduct. The output is connected to the input. During the duration of the gating pulse, the input signal appears at the output. as shown by the lowest waveform. The cathode end of the diode is usually marked in some manner. Diode Voltages  To forward bias a diode, the anode must be more positive than the cathode or LESS NEGATIVE. To reverse bias a diode, the anode must be less positive than the cathode or MORE NEGATIVE. A conducting diode has about 0.6 volts across if silicon, 0.3 volts if germanium. Forward Biased Junction  Bear in mind that like charges repel and unlikes attract. When a battery is connected as shown, the negative terminal pushes negative electrons towards the junction. The positive terminal pushes holes towards the junction. If the voltage is high enough then the barrier will be overcome and current will flow through the junction. There is a voltage across the diode. 0.6 for silicon, o.3 for germanium. The junction is said to be FORWARD BIASED. The P type is the anode of the diode, the N type the cathode, as shown by the diode symbol. The resistor limits the current to a safe level. Diodes come in all shapes and sizes. They are often marked with a type number. Detailed characteristics of a diode can be found by looking up the type number in a data book. If you know how to measure resistance with a meter then test some diodes. A good one has low resistance in one direction and high in the other. There are specialised types of diode available such as the zener and light emitting diode (LED).

A.M. RECEIVER

A.M. RECEIVER TUTORIAL

Most of these blocks are discussed individually, and in more detail, on other pages. See filters, mixers, frequency changers, am modulation and amplifiers. There are signals from thousands of radio transmitters on many different frequencies inducing signal voltages in the aerial. The rf filter selects the desired station from the many. It is adjustable so that the selection frequency can be altered. This is called TUNING. The selected frequency is applied to the mixer. The output of an oscillator is also applied to the mixer. The mixer and oscillator form a FREQUENCY CHANGER circuit. The output from the mixer is the intermediate frequency (i.f.) The i.f. is a fixed frequency of about 455 kHz. No matter what the frequency of the selected radio station is, the i.f. is always 455 kHz. The i.f. signal is fed into the i.f. amplifier. The advantage of the i.f. amplifier is that its frequency and bandwidth are fixed, no matter what the frequency of the incoming signal is. This makes the design and operation of the amplifier much simpler. The amplified i.f. signal is fed to the demodulator. This circuit recovers the audio signal and discards the r.f. carrier. It usually incorporates a diode in the circuit. Some of the audio is fed back to the i.f. amplifier as an AUTOMATIC GAIN CONTROL voltage. This ensures that when tuning from a weak station to a strong one, the loudness from the loudspeaker stays the same. The audio signal voltage is increased in amplitude by a voltage amplifier. The power level is increased sufficiently to drive the loudspeaker by the power amplifier.

F.M. Receiver

F.M. Receiver Tutorial

Most of these blocks are discussed individually, and in more detail, on other pages. See filters, mixers, frequency changers, am modulation and amplifiers. The f.m. band covers 88-108 MHz. There are signals from many radio transmitters in this band inducing signal voltages in the aerial. The rf amplifier selects and amplifies the desired station from the many. It is adjustable so that the selection frequency can be altered. This is called TUNING. In cheaper receivers the tuning is fixed and the tuning filter is wide enough to pass all signals in the f.m. band. The selected frequency is applied to the mixer. The output of an oscillator is also applied to the mixer. The mixer and oscillator form a FREQUENCY CHANGER circuit. The output from the mixer is the intermediate frequency (i.f.) The i.f. is a fixed frequency of 10.7 MHz. No matter what the frequency of the selected radio station is, the i.f. is always 10.7 MHz. The i.f. signal is fed into the i.f. amplifier. The advantage of the i.f. amplifier is that its frequency and bandwidth are fixed, no matter what the frequency of the incoming signal is. This makes the design and operation of the amplifier much simpler. The amplified i.f. signal is fed to the demodulator. This circuit recovers the audio signal and discards the r.f. carrier. Some of the audio is fed back to the oscillator as an AUTOMATIC FREQUENCY CONTROL voltage. This ensures that the oscillator frequency is stable in spite of temperature changes. The audio signal voltage is increased in amplitude by a voltage amplifier. The power level is increased sufficiently to drive the loudspeaker by the power amplifier.

F.M. Transmitter Tutorial

Read the page on Frequency Modulation. The microphone converts sound pressure wave to electrical signals. These audio voltages are amplified by the audio amplifier. The amplified audio is used to control the deviation of the frequency controlled oscillator. The oscillator frequency is at the carrier frequency, in the 88-108 MHz FM band. The low power of the frequency modulated carrier is boosted by the Radio Frequency amplifier. The aerial is driven by the amplifier and produces an electromagnetic wave. Under normal conditions the transmitted signal will travel as far as the horizon