Amplifier U1 uses negative feedback to make sure that the potentials at the inverting (V–) and noninverting (V+) input terminals are equal. In this configuration, V– is directly tied to the local GND; therefore, the potential at the noninverting input terminal is driven to local ground. Thus, the voltage difference across R₂ is the DAC output voltage (VOUT), and the voltage difference across R₅ is the regulator voltage (VREG). These voltage differences cause currents to flow through R₂ and R₅, as illustrated in Figure 9-5.

Figure 9-5 Voltage to Current Conversion

The currents from R₂ and R₅ sum into i₁ (defined in Equation 2), and i₁ flows through R₃.

Equation 2.

Amplifier U2 drives the base of Q1, the NPN bipolar junction transistor (BJT), to allow current to flow through R₄ so that the voltage drops across R₃ and R₄ remain equal. This design keeps the inverting and noninverting terminals at the same potential. A small part of the current through R₄ is sourced by the quiescent current of all of the components used in the transmitter design (regulator, amplifier, and DAC). The voltage drops across R₃ and R₄ are equal; therefore, different-sized resistors cause different current flow through each resistor. Use these different-sized resistors to apply gain to the current flow through R₄ by controlling the ratio of resistor R₃ to R₄, as shown in Equation 3:

Equation 3.

The current gain in the circuit helps allow a majority of the output current to come directly from the loop through Q1 instead of from the voltage-to-current converter. This current gain, in addition to the low-power components, keeps the current consumption of the voltage-to-current converter low. Currents i₁ and i₂ sum to form output current i_out, as shown in Equation 4:

Equation 4.

The complete transfer function, arranged as a function of input code, is shown in Equation 5. The remaining sections divide this circuit into blocks for simplified discussion.

Equation 5.

Resistor R₆ is included to reduce the gain of transistor Q1, and therefore, reduce the closed-loop gain of the voltage-to-current converter for a stable design. Size resistors R₂, R₃, R₄, and R₅ based on the full-scale range of the DAC, regulator voltage, and the desired current output range of the design.