HP 200A
| Industry | Electronics |
|---|---|
| Founded | 1938 |
| Founder | Bill Hewlett, David Packard |
| Headquarters | Palo Alto , United States |
| Products | HP 200A Audio Oscillator |
The HP 200A Audio Oscillator, introduced in 1938, marked Hewlett-Packard's first product, assembled by David Packard in his Palo Alto garage.
This audio oscillator, known for its low distortion, was engineered to test audio equipment and employed the Wien bridge oscillator topology, derived from Bill Hewlett's master's thesis research. It was pioneering in its commercial application of a simple light bulb as a temperature-sensitive resistor within the feedback loop. This clever implementation of a light bulb acted as an affordable and efficient automatic gain control, stabilizing output amplitude and maintaining loop gain close to unity—crucial for minimizing distortion. Prior to this, Larned Meacham had utilized light bulbs in bridge circuits to stabilize oscillators in 1938.
The designation "200A" was chosen to convey the impression of an established enterprise. A tailored version, the HP 200B, was specifically developed for Walt Disney, who acquired eight units for use during the production of Fantasia (1940 film).
Hewlett’s 1939 patent provides an in-depth description of the circuit design.
| HP 200A | HP 200B | |
|---|---|---|
| Frequency range | 35 Hz–35 kHz across 3 ranges X1: 35–350 Hz X10: 350–3500 Hz X100: 3500–35,000 Hz |
20 Hz–20 kHz across 3 ranges X1: 20–200 Hz X10: 200–2000 Hz X100: 2000–20,000 Hz |
| Dial scale | 35–350 | 20–200 |
| Calibration precision | under 2% | under 2% |
| Output power | 1 W into 500 Ω | 1 W into 500 Ω |
| Distortion | below 1% between 35 Hz and 15 kHz | below 1% between 35 Hz and 15 kHz |
| Output impedance | less than 75 Ω below 15 kHz | less than 75 Ω below 15 kHz |
| Power supply | 115 V, 50–60 Hz, 60 W | 115 V, 50–60 Hz, 60 W |
Development
Throughout the 1940s and '50s, newer iterations of the 200A extended frequency coverage significantly. The ultimate version, the model 200CD, offered a range from subaudio frequencies at 5 Hz up to the lower end of the AM radio Long Wave band at 600 kilohertz. From the mid-20th century until the late '90s, the 200CD served as a widely adopted audio signal generator in engineering labs worldwide.
Operation
William R. Hewlett’s implementation of the Wien bridge oscillator integrates a differential amplifier with a Wien bridge arranged in a positive feedback loop connecting amplifier output to its differential inputs. At the oscillation frequency, the bridge achieves near balance with a minimal transfer ratio. The overall loop gain arises from multiplying the amplifier’s high gain by the bridge’s low ratio. Hewlett’s circuit employs two vacuum tubes to provide amplification.
The cathode of tube V1 functions as the amplifier’s inverting input, while tube V2’s control grid serves as the non-inverting input. Apart from R1, R2, C1, and C2, other components form a non-inverting amplifier stage with gain equal to 1+Rf/Rb, featuring high input impedance. The elements R1, R2, C1, and C2 create a bandpass filter that supplies positive feedback precisely at the oscillation frequency. The self-heating resistor Rb introduces negative feedback by increasing resistance with temperature, thus lowering amplifier gain until it stabilizes at just enough level to maintain sinusoidal oscillations without distortion.
When R1=R2 and C1=C2, equilibrium is achieved with Rf/Rb=2 and amplifier gain set at three. At power-up, because the lamp is cold, circuit gain initially exceeds three to initiate oscillations. Additionally, DC bias current flowing through vacuum tube V1 heats the lamp slightly, which helps reduce output amplitude at steady state by supplementing lamp temperature.