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Technical Info

High Quality

Instruments Inc. manufactures quality, high performance, conservatively rated power amplifiers. They are designed to withstand the electrical and mechanical abuses which characterize military and industrial applications. They are immune to damage from such common faults as open or shorted loads, input signal overdrive, excessive power line voltage, and cooling failure.

Our amplifiers are modular in construction for quick repair. All of the semiconductors are mounted on plug-in printed wiring boards (PWBs) which are easily removed from the front of the amplifier.


The most stringent quality control system is useless if operating modes exist which stress any component beyond its limits. Component types and values are selected for conservative operation, even under worst case transient conditions. Our high reliability is a result of conservative design with emphasis on protective circuits.

The result of these efforts is a record of highly reliable amplifier service for more than 30 years. Nearly every amplifier we've built is still in service.

Over-Voltage Protection

The output voltage swing is normally limited by the main power supply voltage and the turns ratio of the output transformer. However, the output voltage may rise to a higher value due to load resonance or an external tuning network failure. We provide Metal Oxide Varistors (MOVs) to protect the amplifier from voltage spikes. Precision voltage limit circuits may be specified for load protection.

Over-Temperature Protection

Under normal conditions, or even normal overloads such as overdrive into a shorted load, the amplifier will not overheat. Thermistors sense the temperature of all components which might overheat. An over-temperature condition shuts down the main supply voltage. Reset is automatic when the overheated item cools down. Excessive over-temperature will trip the main circuit breaker.

Short Circuit Proof

Our switchmode amplifiers sense the voltage drop across each active output transistor (autoprotection). If the load current is excessive, the Output PWB is inhibited for 1 msec.

This protects against both internal and external faults and limits damage if a failure occurs. Current limit is indicated on each PWB, on the front panel, and remotely.

Our linear amplifiers use a fast analog multiplier to calculate the instantaneous transistor dissipation. This information, together with the junction-to-case transient thermal impedance and the case temperature, is used to calculate the junction temperature. The output current limit is then automatically adjusted to safely maximize output power under all conditions of load, drive and temperature. Front panel and remote indication of current limit are provided.

Stable Operation

Reactive loads will not cause our amplifiers to oscillate. The circuitry used in our amplifiers maintains absolute stability with resistive, inductive, and capacitive loads. Each amplifier is extensively tested to reveal any inherent instability.

Drive Reactive Loads

All our amplifiers are designed to drive loads of any power factor. Switchmode amplifiers recycle reactive current between the load and the power supply and are at least 75% efficient.

Broad Frequency Range

Most of our amplifiers use output transformers. Output power, duty cycle and minimum frequency determines the size and weight. The high frequency bandwidth is limited by the primary to secondary coupling factor and the turns ratio. Our careful design of the output transformers and close attention to proper wiring has enabled even the larger sizes of the L series amplifiers to perform above 100 kHz.

The upper frequency limits specified in this catalog apply to the continuous mode on a low impedance tap and refer to the -3 dB point. With each amplifier we provide individual calibration plots of distortion vs. frequency while operating at full rated output power and at -3 dB. The signal input voltage required is also shown.

Operating Tips

The S-series switchmode amplifiers offer compact size and economy. They are particularly advantageous with reactive loads that cause a linear amplifier to have a low efficiency. The L-series linear amplifiers, models L2-L50, are best suited for applications requiring very low distortion. Typical midband THD is less than 0.1%. These amplifiers work best below 100 kHz but have been used to 250 kHz, at much reduced power. The M-series linear amplifiers, M2, M4, M8 are designed for use from 10 kHz to 500 kHz.

Arbitrary Waveforms

When an amplifier must simultaneously transmit two frequencies, the two signals will periodically be in phase so that the maximum amplitude, is the sum. For example, if the two are equal in amplitude, the peak output voltage is doubled. To reproduce this waveform, the amplifier must be rated for four times the power contained in each frequency. In many cases multiple tones can be time multiplexed rather than added. This would allow full use of the amplifier rating.

The amplification of random noise presents a similar problem since it is a mixture of many frequency components. If white noise is to be reproduced, some clipping will occur, since occasional noise amplitudes will exceed the capabilities of the amplifier. It is best if the clipping is done on the input signal. In most cases the signal provided to the amplifier is noise which has a finite peak to average ratio. In these cases all of the peaks can be reproduced if sufficient headroom is available in the amplifier.

The following table shows the percent clipping of Gaussian noise as a function of crest factor rating of the amplifier. Our standard linear amplifiers have a crest factor rating of 3 dB. This allows full reproduction of single frequency component sine waves at the full power rating of the amplifier.

Noise Clipping Crest Factor Average Amplifier
Power Output
16 % 3 dB 0 dB 100 %
11 4 -1 79
8 5 -2 63
5 6 -3 50
3 7 -4 40
1 8 -5 32
0.5 9 -6 25
0.2 10 -7 20
0.04 11 -8 16
0.007 12 -9 13

Sinnema and McGovern, "Digital, Analog and Data Communication", Prentice-Hall, 1986.

Low Frequency Tone Bursts

At the low frequency limit in the specifications, the output transformer core material will be close to saturation. A tone burst has a low frequency component that may saturate the core and current limit the amplifier unless precise modulation procedures are followed. Burst envelope shaping (amplitude modulation) can be used to increase the amplitude over several cycles. Full amplitude can be reached quickly if the first half-cycle is half amplitude. The next half-cycle can be full amplitude. The burst should end with a half amplitude half-cycle of the opposite polarity.

Burst envelope shaping is necessary only in the first octave of the specified bandwidth.

Grounding and Shielding

Operating a large power system in an industrial or shipboard environment is never easy, especially when high voltages, high currents, high frequencies and sensitive, computerized instrumentation are combined.

We have had to learn how to live with the intense EMI conditions within our amplifiers. Likewise, we have learned about our customers' problems in the field.

Our basic philosophy is to provide low inductance wiring for all active circuitry, shield and ground everything, and use differential techniques to reject the remaining common mode voltages. Output wires and load structures can cause capacitive ground currents which return to the amplifier. When the return path includes the amplifier input cables, regeneration (oscillation) may result. We recommend output wiring shields which intercept capacitive currents and return them to the amplifier output connector. We provide a high current ground point.

Input and control wiring should be shielded and grounded at both ends. The outer shield should never be used for signal currents. We use triax cable within our amplifiers because of its excellent near field shielding. Outside the amplifier, twisted shielded pair (twinax) may be acceptable if well spaced from output or power cables.

Ground loops and currents are unavoidable. Capacitively coupled currents will easily circumvent single point grounds and floating circuitry. A more successful system will employ both audio and RF techniques. We will be glad to assist you in your application.

For best results,