“A signal generator produces a defined electrical signal whose characteristics change over time. If these signals appear as simple periodic waveforms such as sine, square or triangle waves, then these signal generators are called function generators. They are often used to check the functionality of a circuit or component. The signal defined by the signal generator is applied to the input of the circuit under test and connected to the corresponding measurement equipment (for example, an oscilloscope) at the output. This allows the user to evaluate the circuit.
A signal generator produces a defined electrical signal whose characteristics change over time. If these signals appear as simple periodic waveforms such as sine, square or triangle waves, then these signal generators are called function generators. They are often used to check the functionality of a circuit or component. The signal defined by the signal generator is applied to the input of the circuit under test and connected to the corresponding measurement equipment (for example, an oscilloscope) at the output. This allows the user to evaluate the circuit. In the past, challenges often included how to design the output stage of a signal generator. This article describes how to design a small and economical output stage built with a voltage gain amplifier (VGA) and a current feedback amplifier (CFA).
Typical signal generators provide 25 mV to 5 V output voltages. To drive loads of 50 Ω or higher, powerful discrete components, multiple parallel components, or expensive ASICs are typically used on the output. It usually has relays inside that allow the device to switch between different levels of amplification or attenuation, thereby adjusting the output level. When the relay is switched on and off as needed to achieve various gains, it will cause intermittent operation to a certain extent. A simplified block diagram is shown in Figure 1.
Figure 1. Simplified block diagram of a typical signal generator output stage.
Using the new amplifier IC as an output stage amplifier can directly drive the load without using internal relays. This simplifies the output stage design of the signal generator and reduces complexity and cost. The two main components of this output form a powerful output stage that provides high speed, high voltage, high current, and a variable amplifier with continuous linear trimming.
Figure 2. Simplified block diagram of a signal generator output stage with VGA.
First, the original input signal must be amplified or attenuated by VGA. The output signal of the VGA can be set to a desired amplitude that is independent of the input signal. For example, if the gain is 10, the output amplitude VOUTis 2 V, the output amplitude of the VGA must be adjusted to 0.2 V. Unfortunately, many VGAs are bottlenecked by their limited gain range. The gain range is rarely greater than 45 dB.
ADI has implemented a programmable gain range of 0 dB to 80 dB on the low power VGA AD8338. Therefore, under ideal conditions, the continuous output amplitude of the signal generator can be set between 0.5 mV and 5 V without additional relays or switching networks. By removing these mechanical components, discontinuous output can be avoided. Because digital-to-analog converter (DAC) and direct digital synthesizer (DDS) components typically have differential outputs, the AD8338 provides a fully differential interface. In addition, with the flexible input stage, any asymmetric input current can be compensated by the internal feedback loop. Meanwhile, the internal node remains at 1.5 V. Under normal conditions, a maximum 1.5 V input signal draws 3 mA through a 500 Ω input resistance. At higher input amplitudes (eg 15 V), a higher resistor in series with the input pins may be required. The resistance value of this resistor should also be such that the maximum input current does not exceed 3mA as when the input voltage is 1.5V.
Many commercial signal generators provide a maximum effective output power of 250 mW (24 dBm) into a 50 Ω (sine wave) load. However, this is often insufficient for applications with higher output power, such as the requirements for testing HF amplifiers or generating ultrasound pulses. Therefore, a current feedback amplifier is also required. The ADA4870 can output ±17 V/1 A from a ±20 V supply voltage. The sine wave can achieve full load output at frequencies up to 23 MHz, making it an ideal front-end driver (output stage) for general-purpose arbitrary waveform generators. To optimize the output signal swing, the ADA4870 is configured for a gain of 10, so the required input amplitude is 1.6 V. However, since the ADA4870 has a ground-referenced input and the AD8338 upstream has a differential output, a differential receiver amplifier should be connected between the two parts for differential-to-single-ended conversion. The AD8130 offers a gain-bandwidth product (GBWP) of 270 MHz and a slew rate of 1090 V/µs, making it ideal for this application. The output of the AD8338 is limited to ±1 V, so the mid-gain of the AD8130 should be designed to be 1.6 V/V. The overall circuit configuration is shown in Figure 3. A 20 MHz bandwidth is achieved at 22.4 V (39 dBm) amplitude and 50 Ω load.
Figure 3. Simplified circuit of a signal generator output stage using a discrete design.
With the combination of a higher power VGA (AD8338), a high power CFA (ADA4870), and a differential receiver amplifier (AD8130), it is relatively easy to build a compact high power signal generator output stage. It offers higher system reliability, longer service life, and lower cost than conventional output stages.
• high speed
AD8130: 270 MHz, 1090 V/µs (G = +1)
AD8129: 200 MHz, 1060 V/µs (G = +10)
• High Common Mode Rejection Ratio (CMRR)
94 dB (min, DC to 100 kHz)
80 dB (minimum, 2 MHz)
• High input impedance: 1 MΩ differential
• low noise
AD8130: 12.5 nV/√Hz
AD8129: 4.5 nV/√Hz
• Input common mode range: ±10.5 V
• Low distortion, 1 V peak-to-peak (5 MHz):
AD8130, -79 dBc (worst harmonic, 5 MHz)
AD8129, -74 dBc (worst harmonic, 5 MHz)
• User adjustable gain
G = +1 no external components required
• Supply voltage range: +4.5 V to ±12.6 V
• Power saving mode
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