Try this transistor circuit

^{simulate this circuit – Schematic created using CircuitLab}

With a standard 10X scope probe on Vout (13pF or so), you'll have about 3 nanosecond (50,000,000 Hertz) bandwidth. Adjust R9 to control the output voltage baseline.

You can increase R3 to 220 or 330 or 430 ohms; at the higher resistance values, the Collector-base capacitance will rise when Vout is near 1.0v and you'll see slower settling. Thus a high-frequency non-linear behavior results (2nd harmonic distortion) and you'll get sum/difference intermodulation. With only 4 bits, I doubt this will be a problem for you. But you might scale up a few more resistors, to 6 or 8 bits, and feed with pre-canned sum-of-sin waveforms and then examine the FFT on a scope or Spectrum analyzer.

Try this transistor circuit

^{simulate this circuit – Schematic created using CircuitLab}

With a standard 10X scope probe on Vout (13pF or so), you'll have about 3 nanosecond (50,000,000 Hertz) bandwidth. Adjust R9 to control the output voltage baseline.

You can increase R3 to 220 or 330 or 430 ohms; at the higher resistance values, the Collector-base capacitance will rise when Vout is near 1.0v and you'll see slower settling. Thus a high-frequency non-linear behavior results (2nd harmonic distortion) and you'll get sum/difference intermodulation. With only 4 bits, I doubt this will be a problem for you. But you might scale up a few more resistors, to 6 or 8 bits, and feed with pre-canned sum-of-sin waveforms and then examine the FFT on a scope or Spectrum analyzer.

Performance Enhancement: if you can bias the bottom of the 2 resistors: R1 and R9, to -0.2 volts, then your linearity will improve, likely detectable for large #bits. Note the loading on the Logic Input lines is not consistent, and this also produces NonLinearities.

Using differential current steering, perhaps with bipolar current sources and diode-switches used to steer, reduces the Nonlinearity. At some point, you have expensively built a DAC08 from Precision Monolithics Corp, but with 20MHz to 50MHz bandwidth. Examine that datasheet.

http://www.ti.com/lit/ds/symlink/dac0800.pdf

Try this transistor circuit

^{simulate this circuit – Schematic created using CircuitLab}

With a standard 10X scope probe on Vout (13pF or so), you'll have about 3 nanosecond (50,000,000 Hertz) bandwidth. Adjust R9 to control the output voltage baseline.

Try this transistor circuit

^{simulate this circuit – Schematic created using CircuitLab}

With a standard 10X scope probe on Vout (13pF or so), you'll have about 3 nanosecond (50,000,000 Hertz) bandwidth. Adjust R9 to control the output voltage baseline.

You can increase R3 to 220 or 330 or 430 ohms; at the higher resistance values, the Collector-base capacitance will rise when Vout is near 1.0v and you'll see slower settling. Thus a high-frequency non-linear behavior results (2nd harmonic distortion) and you'll get sum/difference intermodulation. With only 4 bits, I doubt this will be a problem for you. But you might scale up a few more resistors, to 6 or 8 bits, and feed with pre-canned sum-of-sin waveforms and then examine the FFT on a scope or Spectrum analyzer.