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Analog Electronics Trainers

Null, resonate, oscillate

Null: the Anderson bridge

A bridge does not measure a value. It compares the unknown against known components and detects the condition where the detector reads nothing. That is the source of its accuracy: a null detector needs sensitivity, not calibration, so the precision of the result comes from the known components rather than from any meter.

The Anderson bridge measures inductance against a standard capacitor, with an extra arm that makes balance achievable for coils that the simpler bridges struggle with. Three practical points decide whether the experiment works in a teaching laboratory:

• The detector must be sensitive. Headphones, a tuned amplifier or a sensitive meter - the narrower its bandwidth, the deeper the apparent null.
• The source frequency must be stable, because balance is frequency-dependent. A drifting generator produces a null that will not stay put, and a class that concludes the bridge is faulty.
• The sharpness of the null depends on the coil's Q, and a low-Q coil gives a broad, shallow null. Tell students this before they start, because otherwise the first group with a resistive coil spends the session hunting for a sharp balance that cannot exist - and learns the wrong lesson from a correct measurement.

Resonate: series and parallel resonance, and where Q comes from

The resonance board gives both forms, and the contrast is the lesson: at resonance a series L-C-R circuit has minimum impedance and maximum current; a parallel circuit has maximum impedance and minimum line current. Same resonant frequency, opposite behaviour, and every filter and tuned amplifier a student meets later is one of these two.

The measurement worth the session is Q from the bandwidth: find the resonant frequency, find the two frequencies at which the response has fallen to the half-power points, and Q is the resonant frequency divided by the difference between them. It turns Q from a symbol into something measured.

And the trap that catches almost every class, which we would rather print here than leave in a footnote: the instruments change the circuit. A signal generator with a low output impedance damps a parallel resonant circuit, and a meter or oscilloscope probe across the circuit loads it further. Both reduce the measured Q below the circuit's own Q, and the result is a perfectly careful measurement of the circuit plus the instruments. The remedies are to use a high-impedance probe, to couple the generator loosely rather than directly, and - best of all - to repeat the measurement with the probe moved and watch the number change, which teaches more about measurement than the resonance curve itself.

Oscillate: Hartley, Colpitts and why the Clapp exists

All three circuits on AET-0002 satisfy the same condition - round the loop, the gain is unity and the phase shift is zero - and all three differ only in how the feedback is tapped off the tuned circuit:

• Hartley: the inductor is tapped. Easy to get going, easy to tune over a range by varying the capacitor.
• Colpitts: the capacitance is split into two in series and the junction provides the tap. Generally better waveform purity and the arrangement found inside most practical oscillators.
• Clapp: a Colpitts with an additional capacitor in series with the inductor. This is the one worth teaching carefully. That extra capacitor is small compared with the other two, so it dominates the resonant frequency - which means the transistor's own internal capacitances, which drift with temperature and bias and vary between devices, have much less influence on where the oscillator sits. The Clapp exists for frequency stability, and having all three on one board is what lets a student see that rather than be told it.

The experiment that uses the board properly: measure each oscillator's frequency, compare it with the value calculated from the components, then warm the transistor gently - a finger will do - and watch which circuit's frequency moves least. That single comparison justifies the board.

What is not in the box

These three trainers are passive and active circuit boards. The instruments around them are the larger part of the real cost, and a quotation for the boards alone is not a quotation for a working laboratory. You will need:

• A signal generator covering the frequency range of the experiments, with a stable output - the resonance and bridge experiments both depend on it.
• An oscilloscope of adequate bandwidth, with a high-impedance probe, for the oscillator waveforms and for the resonance response.
• A sensitive null detector for the bridge - headphones or a tuned amplifier.
• A frequency counter, or an oscilloscope you trust for frequency, for the oscillator stability comparison.

Tell us what you already own and we will quote the gap rather than the catalogue.

What to tell us when you enquire

• The experiments your syllabus names, and whether bridge measurement of inductance is required specifically - several syllabuses name a particular bridge.
• Whether the boards must have accessible test points and the ability to be wired up by the student, or whether fixed internal wiring is acceptable.
• What instruments the laboratory owns, with the oscilloscope bandwidth and the generator range.
• Group size and session length.
• Supply voltage and frequency, and destination country.

Related ranges: basic electronics trainers and electronics lab trainers for the main analogue range, digital and analog trainer kits, digital electronics lab trainers for the digital half of the syllabus, and power electronics lab trainers. The full training kits and electronics lab trainer kits categories list the rest.

Frequently asked questions

Why can we not get a sharp null on the bridge?

Usually the coil, not the bridge. The sharpness of a null depends on the Q of the inductor being measured, and a resistive, low-Q coil gives a broad and shallow balance point - that is the correct behaviour, not a fault. After that, check the detector's sensitivity and bandwidth, and check that the source frequency is stable, because balance is frequency-dependent and a drifting generator moves the null while you are looking for it.

Why is our measured Q lower than the calculated value?

Because the instruments are part of the circuit. A low-impedance signal generator damps a parallel resonant circuit, and a probe across it loads it further, so what you measured is the circuit plus the instruments. Use a high-impedance probe, couple the generator loosely rather than directly, and demonstrate the effect deliberately by moving the probe and watching the number change. That demonstration is worth more than the resonance curve.

What is the difference between the Hartley, Colpitts and Clapp oscillators?

Only how the feedback is tapped from the tuned circuit. Hartley taps the inductor; Colpitts splits the capacitance and taps the junction; the Clapp is a Colpitts with an extra, smaller capacitor in series with the inductor, which dominates the frequency and therefore reduces the influence of the transistor's own capacitances. The Clapp is the stable one, and the point of having all three on a board is that a student can measure that rather than read it.

How do we demonstrate oscillator frequency stability?

Measure each oscillator's frequency, then warm the transistor slightly - a fingertip is enough - and measure again. The circuit whose frequency moves least is the one whose frequency is set by components rather than by the device, which is the Clapp. Keep the heating gentle and consistent between circuits, and record the frequency with a counter rather than by eye on a timebase.

Do these boards come with experiment manuals?

Ask per item and get it in writing. For boards like these the procedure, the theory, the observation table and a worked example are a substantial part of the value, and they are what determine whether the board is still in use after the member of staff who understood it moves on. If no manual is supplied, budget staff time to write one.

Is three products the whole of your analogue range?

No - this category holds three, and we have said so rather than implying otherwise. The main analogue range is in the basic electronics trainers and electronics lab trainers categories linked above, with a wider set of amplifier, oscillator, filter and power supply experiments. If you send us your full analogue syllabus we will quote across all the categories in one reply.

Key takeaways

• A bridge measures by null, so its accuracy comes from the known components - the detector needs sensitivity, not calibration.
• A low-Q coil gives a broad null. Tell students first, or they will hunt for a balance that cannot exist.
• Q is measured from the bandwidth between the half-power points - that is what turns Q from a symbol into a measurement.
• Your instruments damp the circuit. A low-impedance generator and a loading probe both reduce measured Q; demonstrate it rather than ignoring it.
• The Clapp oscillator exists for stability, because one small series capacitor dominates the frequency instead of the transistor.
• Budget the instruments. Generator, oscilloscope with a high-impedance probe, null detector and counter are the larger part of the real cost.

About this page

Ray Export is a manufacturer and exporter of laboratory, scientific and educational equipment based at Plot No. 449, HSIIDC Industrial Area, Saha, Ambala, Haryana 133104, India. Enquiries: [email protected], +91-9817319615.

Written by Palak Mall, Senior Content Writer. Technically reviewed by Arvind Kumar, Lab Equipment Specialist, 12+ years in laboratory and testing instrumentation. Last updated: 17 October 2026.

This page is deliberately short because the category holds three records; padding it to the length of our larger category pages would serve nobody. Notes published rather than hidden: the product names carry the padding phrase "- Analog Electronics Lab Trainer For Vocational Training And Didactic...", which is keyword padding rather than product information and has truncated the names; we are removing it. One record's name reads "Hartley s, Colpitt s, Clapp" because apostrophes were stripped on import - the circuits are properly the Hartley, Colpitts and Clapp oscillators, and we print the correct forms here because those are the words a specifier searches for. A three-product category named "analog electronics trainers" understates the range, and we have recommended it be merged with the main analogue categories. All specifications are confirmed in writing on the quotation; for certificates and standards, contact us for the latest copies of the relevant certificates before tender submission.