Circuits

Audio Preamplifier

This audio preamplifier uses discrete components in the first three stages which together provide a voltage gain in excess of 500. The circuit is based on low-noise, high-gain transistors in TO18 packages.

Low-noise metal oxide film resistors provide bias, collector load and emitter stabilizing resistors.

Switched Mode Power Supply

This switched-mode power supply provides regulated d.c. outputs of +5V at 2A and +12V at 1A.

Fast-switching power diodes provide the necessary switching action together with miniature inductors and electrolytic filter capacitors.

The switching action is controlled by an LSI integrated circuit which incorporates its own internal voltage reference.

Logic Controller

This logic controller uses standard '74LS' series logic gates.

The 74LS04 and 74LS05 chips are both hex inverters. Their 14-pin DIL packages each contain six separate inverting gates.

The 74LS02 is a quad two-input NOR gate and the 74LS08 is a quad two-input NAND gate.

All of these logic gates operate from a common +5V supply. The +5V and 0V power rails are the slightly wider copper tracks on the PCB.

The chips operate using conventional TTL logic levels which are defined as:

• 'logic 0' - or 'off' - a voltage of less than 0.8V

and:

• 'logic 1' - or 'on' - a voltage of between +2V and +5V

Crystal Oscillator

This crystal oscillator circuit provides the basic timing reference signal for a complete microprocessor system. The oscillator uses a 12MHz quartz crystal mounted in a sealed metal casing.

The output of the oscillator drives a 74S04 hex inverter and this provides a square wave clock signal at 12MHz for the microprocessor.

Several discrete transistor stages can also be seen. One is based on a TO18 encapsulated device and the other uses a TO92 cased transistor.

Also note the polystyrene timing capacitors and metal oxide bias resistors.

VLSI Disk Controller

This VLSI - very large scale integration - controller chip is used in the graphics card of a modern PC. The device is a 90C31 manufactured by the Western Digital Corporation and it replaces a very large number of individual logic devices.

The narrow copper tracks convey the digital input and output signals used by the device whilst the thicker tracks are ground (or 0V) and +5V.

The small holes on the surface of the PCB are 'plated-through' and convey signals to one of a number of printed circuit track planes inside the multi-layer circuit board.

Power Controller

This power controller circuit uses a number of plastic encapsulated power transistors to control the current flowing in its four load circuits.

Each of the TIP31 transistors is rated at 60V maximum collector to emitter voltage and 3A maximum collector current.

Carbon film resistors are used to limit the base current of each of the emitter-follower stages.

Power Amplifier

This audio frequency power amplifier forms the modulator stage in a mobile transceiver.

The power amplifier provides an output of 10W and it uses two TO66 encapsulated power transistors operating in class-B push-pull. The output is coupled via a transformer into the final class-C radio frequency power amplifier.

VHF RF Power Amplifier

This VHF RF power amplifier is the final stage in a mobile transceiver.

The stage uses a single transistor operating in class-C mode and it produces an RF output of about 8W into a 50Ω antenna load.

The three coils form part of the input and output tuning and matching networks whilst the solid-dielectric preset variable capacitors are used during the alignment procedure.

High Gain Wideband Amplifier

This high-gain wideband amplifier is based on an LF412 dual operational amplifier.

The stage gain is adjustable by means of a pre-set resistor in the feedback path.

The stage is screened from other adjacent amplifier stages by means of an earthed [or grounded] metal screening compartment which helps to avoid instability and stray signal pickup.

Op. Amp Audio Amplifier

This audio filter is based on a single TLO71 operational amplifier stage.

Capacitors connected in the feedback and input circuits form a frequency selective network together with the feedback and input resistors. The combined effect of these components is that the frequency response of the amplifier becomes limited to a narrow band of frequencies - this is known as the passband.

The presence of a signal within the amplifier's passband is detected by means of two germanium signal diodes.

The radial lead electrolytic capacitor provides decoupling of the supply.

UHF Construction

This RF amplifier stage shows some typical aspects of circuit construction at VHF and UHF frequencies.

The circuit uses a single low-noise RF transistor with bias potentials provided by three metal film resistors.

The input to the RF stage comprises a tuned band-pass filter with each stage electronically tuned by a variable capacitance diode.

Fixed tuning is provided by three miniature ceramic dielectric preset capacitors and the tuning inductors comprise parallel "striplines" on the printed circuit.

The d.c. tuning potential for the variable capacitance diodes is fed via three miniature resistors and printed circuit inductors.

Surface Mounted Components

Surface mounted components allow circuits to be built in the smallest possible space. This picture shows some typical surface mounted resistors, capacitors, and inductors. The device marked 'AY6' is a surface mounted transistor (note the three soldered connections).

Surface Mounted IC's

Surface mounted components allow circuits to be built in the smallest possible space.

This close-up picture of the motherboard from a laptop computer shows some typical surface mounted integrated circuits.

The two conventional axial lead components will give you some idea of the scale of miniaturization!